How do you solve this ?
Why is the ans C not B ?

Explanation:

Draw a free body diagram of the toolbox.  There are two forces:

Weight force mg pulling down,

and applied force F pulling up.

Sum of forces in the y direction:

∑F = ma

F − mg = ma

45 N − 15 N = (3 kg) a

a = 10 m/s²

The answer should be B.  It's possible the answer key has a mistake.

be subject to a force andchange its motion