Answer:
a. Comma
Step-by-step explanation:
A clause is a group of words with a subject and a verb. There are two types of clauses: independent (when it expresses a complete thought, and thus, it can stand on its own) and dependent (when it does not express a complete thought).
When a dependent clause comes before the independent clause, the clauses must be separated by a comma. Take a look at the following examples (the dependent clauses are underlined):
Even though I loved him, I had to stop calling him.
Since you won't go to the museum either, I will stay home.
After you finish your homework, go to the kitchen and wash the dishes.
When my mom comes home from work, she sits on the sofa to read books.