Read the compound sentence.

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.