She is keen ___ learning new skills.

The correct option is B. on. The phrase 'keen on' is an idiomatic expression in English, meaning to be very interested in something or enthusiastic about doing something. In this context, 'She is keen on learning new skills' correctly conveys her enthusiasm for acquiring new knowledge.

• A. at: 'Keen at' is not a standard idiom in this context; 'good at' is common, but not 'keen at'.
• C. of: 'Keen of' is grammatically incorrect and not used to express enthusiasm.
• D. for: While 'keen for' can sometimes be used to express a strong desire for something (e.g., 'keen for a promotion'), 'keen on' is specifically used when referring to an activity or interest.