In a parallelogram opposite angles are equivalent so B=D and A=C.
If B=D, 3n + 20 = 6n - 25; add 25 to both sides
3n + 45 = 6n; subtract 3n from both sides
45 = 3n; divide both sides by 3
15 = n; substitute 15 for n to find the value of angle B;
3(15) + 20 = B
45 + 20 = B
65° = B
Angles C and B are supplementary, which means that when added they equal 180°
so 180 = 65 + C
115 = C
Your answer is choice B