Answer:
Astronomy is the right answer.
Explanation:
In this case, the relative clause "which is not a required course" is modifying the noun "astronomy". Relative clauses are ALWAYS after the noun they modify; this is because if not, it would create a "dangling modifier" (the reference is not clear). In addition, this clause is a "non-defining" relative clause because it can be erased and the sentence still make sense.
