Answer:
A) a phase
Step-by-step explanation:
There are two clauses, separated by a comma. So, you can rule out B and C, since they both include only one clause.
An independent clause can stand alone, without the other part of the sentence. "I know college is stressful," is an independent clause. "But I am confident you will succeed," is not, because it is not a complete thought without the rest of the sentence. So this rules out D since there is only one independent clause.
This makes the answer A, a phrase.