Which word completes the sentence: 'Thanks for following up _______ it.'

• A. In
• B. At
• C. With
• D. On

Answer: (D)

When you use the phrasal verb follow up, the preposition you choose shows what is being pursued. Here, the goal is addressing a matter or topic, so the natural pairing is on. Thanks for following up on it is a common, clear way to acknowledge that someone has taken action regarding that issue.

Other options don’t fit the same sense. In it isn’t the usual collocation for topics or issues. With would imply following up with a person (as in following up with you) rather than addressing the matter itself. The sentence uses the matter, so on is the best choice.