Answer:
The correct way to punctuate the sentence is just as the following sentence: Dr. Burke wanted to trust her assistant with the secret formula, but she was afraid her assistant was a spy.
Explanation:
The sentence above is the correct choice because it contains a comma before the coordinating conjunction. When two independent clauses are joined with a coordinating conjunction, a comma is required before the conjunction. Without the comma, the sentence is a run-on sentence.
