Larger is in the comparative form.
Positive is large.
Superlative is largest.
Comparative degree of comparison compares only two nouns. In the sentence, Jupiter and Mercury are compared. So the answer is B.
You are trying to convince the principal with your essay. She doesn't know which to build, so your essay should tell her your opinion. You might want to persuade the students, but the principal will make the final decision, so you need to convince her first, :)