Learn

Hi Belly,
no, I don't think it is very difficult. You just need to use a dictionary for learners. If you don't have one, you can use some online versions, there are the most popular ones on the net, available for free. Try the Oxford Advanced Learner's Dictionary, or the Longman Dictionary of Contemporary English.
You'll see that after the verb "learn", you use a "to" form of the verb. So C should be the answer.

By the way, I just thought that in certain contexts... If you have to take an exam about sports, you have to study those sports, the rules, training methods, etc. So I was thinking that if you are studying a book about sports, you could go on learning swimming (= subject) after learning skating (= subject that comes before in alphabetical order). LOL, is this possible?

Just my opinion, Belly.