Final answer:
The correctly punctuated compound sentence is D: Running a 5K race would be a great accomplishment, and exercise will improve my health.
Step-by-step explanation:
In the given options, the correctly punctuated compound sentence is: D. Running a 5K race would be a great accomplishment, and exercise will improve my health.
A compound sentence consists of two independent clauses joined by a coordinating conjunction (and, but, or, etc.) and a comma. In option D, the two independent clauses, 'Running a 5K race would be a great accomplishment' and 'exercise will improve my health', are joined by the coordinating conjunction 'and' with a comma before it.