Answer:
a.) Gerund
Step-by-step explanation:
A gerund phrase is verbal a phrase consisting of a gerund (which it's a verb root + "ing") and modifiers or objects of the gerund, whose function is to act as a noun.
In the given sentence, "Preparing for the exam" is a gerund phrase because it has a gerund "Preparing" plus their modifiers and object " for the exam", and it also works as a noun and the subject of the sentence.