Singular.

Much of the language, and much of any style guide, is subjective. Some rules are absolute. Other guidance is just that: guidance.

Sometimes, there's only one acceptable spelling by anyone's definition. Horse, for example, when meaning the animal. Sea, when meaning the body of water. Other examples: Spelling. Words. Writing. Editing. Frustration. Inconsistency. Welcome. To. My. Life.

Other times, a word may have varying spellings depending on the source. Webster's New World College Dictionary prefers the two-word sea horse. Merriam-Webster prefers the one-word seahorse. While AP style and AP writers generally follow Webster's New World College Dictionary, we also have exceptions based on usage or other reasons.

In this case, AP usage over time has preferred seahorse. Given that it's an acceptable spelling recognized by an established and respected dictionary, I'm willing to accept that one-word version as one of our exceptions to our primary dictionary.

Technically, it's: Day One through Day Nine, then Day 10, Day 11, Day 12, etc.

That said, and as our numbers guidance continues to be under review, I'd also say this: If you're listing a number of days, I'd probably go with all numerals for consistency and for what makes most sense to your readers. Day 1, Day 5, Day 50, etc.

The Children's Online Privacy Protection Act. (Note: it's Privacy Protection, not Protection Privacy)

The District 4 commander, Adm. Keith Smith, presided over the ceremony.

Yes, that's right.

I'd make it a master's in higher education administration.

No hyphen needed in that use.

I'm not seeing a typo. The entry is intended to point out the difference between to flounder (move clumsily or jerkily; flop about) and to founder (bog down, become disabled or sink)

So to rephrase the example: The ship moved clumsily or jerkily in heavy seas for hours, then sank.

Or better yet, avoid using either word!

A flounder is a fish; to flounder is to move clumsily or jerkily, to flop about: The fish floundered on land.

To founder is to bog down, become disabled or sink: The ship floundered in the heavy seas for hours, then foundered.

Either is fine.

With or without the quote marks is fine. I think it's a little clearer to use them.

Either is fine, depending on your preference. I'd use both hyphens.

We'd use Dopp kit as as a trademark.

He filed for Chapter 13 bankruptcy protection, or he filed for bankruptcy (and explain later)

Webster's New World College Dictionary shows this for working as a noun: –n. 1 the act or process of a person or thing that works
And this for remote as an adjective: adj. –mot'er, –mot'est [[ME < L remotus, pp. of removere, to REMOVE]] 1 distant in space; far off; far away 2 far off and hidden away; secluded 3

So: Remote working is fine. Remote work also is fine, as is working remotely. On this one, it comes down to the stylistic preferences of you and the writer.

FWIW, I prefer working remotely.

C ration, according to Webster's New World College Dictionary:

C ration  [[< c(ombat)]] a canned ration used in the field in WWII and later

We use numerals for all ages.
Children ages 4 and under.

You're looking at the Webster's New World College Dictionary entry and spellings, which you get as part of your Stylebook Online subscription. The Stylebook itself doesn't have a chili con carne entry. But we do say, in the chile, chiles entry: The meat- and/or bean-baed dish is chili. So we'd call it chili con carne as well.


Webster's New World

(chil´ē kän kär´nē)

[[< MexSp chile con carne, lit., red pepper with meat]] a highly seasoned Tex-Mex dish of ground or chopped beef, chilies or chili powder, and other spices, and, often, beans and tomatoes

It depends on what you mean by the entire description. First off, what does biggest mean here? Is it meant to compare this challenge with a past one of, for instance, 20 hours? Or is the fundraising goal higher? Or something else? Was there a past 24-hour challenge that wasn't online?

A quick check online tells me that you've had similar online challenges, lasting 24 hours, in the past. If that's the case, this is correct:  It is the university's biggest 24-hour online giving challenge.

I'm still not clear on what biggest means here, but this construction works.

Good luck with the challenge!

We've been calling it a new virus or a new coronavirus, on first reference, then giving details later.

No. Use lowercase for those.

I'd uppercase the award category as well.

Yes.

You could delete the period after tell it in the quote. Or you could rewrite. If your thought is to rewrite, that's probably your best bet.