There wasn't very much interest ____ the idea. Which word completes the sentence?

• A. In
• B. On
• C. About
• D. For

Answer: (A)

The main idea here is how we connect the noun “interest” to what it’s about. When we talk about how much people care about or are curious about something, we use the preposition in to show the topic or object of that interest. So "interest in the idea" means people care about or are curious about the idea.

Other prepositions don’t fit the natural pattern. “Interest on” is used for financial reasons (like interest on a loan), not for topics. “Interest about” isn’t common English for this meaning, and “interest for” isn’t the usual way to express what someone is interested in.

So the sentence sounds natural with “in the idea” because it correctly links interest to its target.