I wasn't too keen ______ going to the party.

• শূন্যস্থানে সঠিক উত্তর হবে - on.
- Complete sentence:  I wasn't too keen on going to the party.

• Keen 
English Meaning: Wanting to do something or wanting something to happen very much
Bangla Meaning: (কোনো বিষয়ে উৎসাহ বা অনুভব) তীব্র; গভীর।

• Prepositions used with Keen: 

•  keen on doing something
- I wasn't too keen on going to the party.

• keen to do something
- John was very keen to help.

 • keen for somebody/something to do something
- His father was keen for him to go to university.
- He is keen for the deal to proceed.

Source: Oxford Learner's Dictionary and Accessible Dictionary by Bangla Academy.