Question

Dr. Burke wanted to trust her assistant with the secret formula but she was afraid her assistant was a spy.

Which is the correct way to punctuate the sentence?

Dr. Burke wanted to trust her assistant with the secret formula; but she was afraid her assistant was a spy.
Dr. Burke wanted to trust her assistant, with the secret formula: but she was afraid her assistant was a spy.
Dr. Burke wanted to trust her assistant with the secret formula but, she was afraid her assistant was a spy.
Dr. Burke wanted to trust her assistant with the secret formula, but she was afraid her assistant was a spy.

Answer 1

the best way to punctuate the sentence would be your last sentence

Answer 2

The correct way to write the sentence that you provided is the last way. “Dr. Burke wanted to trust her assistant with the secret formula, but she was afraid her assistant was a spy.”