This would be the Associative Property of Addition, or option C. In an addition problem, the order that you solve the problem in does not matter, as you are simply adding them together.
summation from k equals 1 to 5 cosine k pi
= Cosine 1*∏ + Cosine 2*∏ + Cosine 3*∏ + Cosine 4*∏ + Cosine 5*∏
= -1 + 1 + -1 + 1 + -1
= -1
The slope is undefined, because the x values are the same.
