Dynamic stretches are part of an effective warm-up because they can

a) lubricate the muscles used in the activity

b) decrease cardiovascular difficulty in the activity

c) expand lung capacity for the activity

d) build endurance for the activity

Dynamic stretches are part of an effective warm-up because they can help A. lubricate the muscles used in the activity. 

Dynamic stretching produces the greatest reduction in sprint time or the greatest increase in running speed.It is an integral part of any warm-up in sports or other physical activities.
Hence,
Dynamic stretches are part of an effective warm-up because they can helplubricate the muscles used in the activity.
So the best option is: 
A) lubricate the muscles used in the activity