Answer:
C. The old man opened the window; he took a deep breath of the crisp fall air.
Step-by-step explanation:
A run-on sentence can be corrected by inserting a semicolon between the two sentences. A semicolon is known as a soft period and joins two complete sentences closely related in thought.
Example:
Run-on sentence: The storm winds have increased, they are now gusting at up to 70 miles per hour.
Correction: The storm winds have increased; they are now gusting at up to 70 miles per hour.