Final answer:
The correct way to punctuate the sentence is using a semicolon to separate the two independent clauses that are closely related.
Step-by-step explanation:
The correct way to punctuate the sentence is:
Dr. Burke wanted to trust her assistant with the secret formula; but she was afraid her assistant was a spy.
In this sentence, a semicolon (;) is used to separate two independent clauses that are closely related. The semicolon is used to indicate a stronger pause than a comma would provide, but not as strong as a full stop (period). By using a semicolon, the sentence maintains its flow and avoids any confusion.
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