They are not interested ___ our proposal.

This question tests the correct preposition to use with the adjective “interested.” Prepositions often follow certain adjectives to form idiomatic phrases.

• The correct answer is in. The standard and grammatically correct phrase in English is “interested in.” This means having or showing curiosity or concern about something. For example, one is interested in a topic, a book, or a proposal.
• Using “by” (option C) with “interested” is less common and usually implies being affected or intrigued by something, rather than having a general interest.
• Using “on” (option D) is grammatically incorrect in this context. One might work on a proposal or focus on it, but not be interested on it.