Which free-body diagram shows an object that will accelerate?

a. diagram a
b. diagram b
c. diagram c
d. diagram d

Diagram C

Explanation:

When a body is under the influence of 2 or more forces, the effect of all forces may be replaced by a unique force (resulting force), with the same result as all original forces.

In case you have different forces with the same direction but different sense, all forces may be substituted by one resulting force, though the module of this resulting force will be equal to the absolute value of the substraction of the modules of the individual forces

For case C, you should check: 8 N (force with right sense) + 4 N (force with right sense) - 4 N (force with left sense) would give us a resulting force of 8 N (with right sense), which will accelerate the body to the right