We have ∠C + ∠D = 180º, because C and D are supplementary.
Then, if ∠C = 116º, we have:
∠C + ∠D = 180º
116º + ∠D = 180º
In order to solve this equation, we can apply the same algebraic operations on both sides of the equation, until we isolate the variable ∠D, and find its value:
116º + ∠D = 180º
116º + ∠D - 116º = 180º - 116º
∠D = 64º
Therefore, option C is correct.
