I'm really keen ___ going to the park with my daughter.

• A. On
• B. In
• C. For
• D. To

Answer: (A)

The key idea here is the fixed expression used to show enthusiasm for an activity: keen on + the -ing form. When you’re excited about doing something, you say you’re keen on doing it, so in this sentence “going” is a gerund that follows the preposition “on.” That combination—keen on going—makes the meaning natural and correct: you’re really enthusiastic about the idea of going to the park with your daughter.

Other prepositions don’t fit this natural pattern. “In” isn’t used with this meaning, and “keen in” sounds odd for expressing personal interest. “For” isn’t the usual choice when talking about enjoying an activity. A different construction would be “keen to go,” where you’d use the infinitive after “to,” but that changes the form of the verb from a gerund to an infinitive, which isn’t compatible with “going” in this sentence.