There is no context for the sentence. It is just one sentence from an online grammar exercise. I wrote "covenant" and got no point. I have only one more chance.
First, I must apologize for not having read the original sentence completely-- I did not see 'every month' originally. This should be a warning to all posters to include their question material in the body of the post, where it can easily be read and referred to.
Now:
The students covenant to pay tuition every month. -- This is the expected answer and both coven