Mary is a ... student than Lucy.

The correct answer is 'better'. Here's why: * The sentence is comparing Mary's abilities as a student to Lucy's. When comparing two things, we use the comparative form of an adjective. The comparative form of 'good' is 'better'. * 'The best' is the superlative form, used to describe something that is the highest degree of quality (e.g., Mary is *the best* student in the class). * 'Best' is also a superlative, and doesn't fit the grammar of the sentence. * 'Gooder' is not a word. Therefore, the only grammatically correct and meaningful option is 'better'. Answer: better