First Certificate in English (FCE) 2 Practice Test 2026 - Free FCE Practice Questions and Study Guide

The main idea is how adjectives partner with prepositions to express interest in an activity. With the adjective keen, the natural pairing when talking about enjoying doing something is keen on followed by a gerund. So, keen on playing fits because it shows enthusiasm for the activity of playing.

If you used a different preposition, it wouldn’t sound natural in this context; and if you tried to use an infinitive, you’d say keen to play instead, without the -ing form. Here, since the sentence has playing (a gerund), the correct choice is the preposition that pairs with keen plus a gerund.