Final answer:
Option C uses a semicolon correctly by connecting two closely related independent clauses without a coordinating conjunction.
Step-by-step explanation:
The sentence that uses a semicolon correctly is option C: 'Mowing the lawn will take an hour; unless you hurry.'
A semicolon is used to connect two closely related independent clauses without a coordinating conjunction. In this sentence, the two independent clauses 'Mowing the lawn will take an hour' and 'unless you hurry' are connected by a semicolon.
Example: 'I love to swim; it is a great way to exercise.'
Learn more about Semicolon usage