He warned her _______ if she didn't think of something in the next twenty-four hours, there would be trouble.

• A. what
• B. which
• C. when
• D. that

Answer: (D)

The main idea here is how to report the content of a warning. After verbs like warn, English often uses a clause that states exactly what was said. That introduces the whole statement the person warned about. Here, the content of the warning is “if she didn't think of something in the next twenty-four hours, there would be trouble.” So you need a word that can introduce that full idea as a single clause, and that word is that. It links the verb of saying with the reported content.

The other options wouldn’t fit the structure: what would start a different type of clause and isn’t the way to express the content of a warning here; which and when don’t introduce a warning content in this context.