The correct answer is letter "C) Swimming is my favorite form of exercise; it gives me energy".
Explanation:
Semicolon may be used to separate and/or organize independent clauses and/or headings and subheadings. In this case of letter C, the semicolon is used to better organize the sentence by dividing the first main independent clause ("Swimming is my favorite form of exercise") from the second clause which expresses a secondary idea ("it gives me energy"). All the other options are lacking in some way.
