The words chosen to create meaningful dependent clauses are the following ones:
a) Although Marissa likes to bake, she doesn't like to cook (this conjunction is used to make the main statement in a sentence seem surprising)
b) I have an idea that I think you will like (the complementizer introduces the noun-complement clause attached to the noun "idea")
c) If we drive slowly, we will find it (the conjunction used to introduce a conditional clause)
d) French, which we also had last year, is my hardest subject (a relative pronoun, which in this case, introduces a non-essential relative clause)