Answer:
Strolling in the park on a summer afternoon
Step-by-step explanation:
If the choices are
A. Strolling in the park on a summer afternoon.
B. Talking on the phone is her favorite pastime.
C. The Carters enjoy vacationing at the lake.
D. In December, we usually take a skiing trip.
because the other options are complete sentences. Option A is slightly complete but not fully.
Hope this helps :)