Perimeter of triangle = 3(side)
P = 3(s)
12 = 3s
12/3 = s
4 = s
I think there is a typo in your post.
The question should be: WHICH ONE OF THE FOLLOWING IS A POSSIBLE SIDE OF THE TRIANGLE?
In that case, 4 cm is the answer. However, 4 cm is not listed among the choices. So, choice d is the answer.