In a parallegram, opposite sides are always congruent.
Therefore, we can say that AD = BC since they are opposite sides.
With this relationship of AB and BC, let's find x.
AD = BC
25 = 3x + 10
3x + 10 = 25
3x = 25 - 10
3x = 15
3x/3 = 15/3
x = 15
Therefore, x = 15
The answer is CHOICE D.