Which equation is represented by the intersection of the graphs below?

a. sec x=-1
b. tan x=-1
c. sin x=-1
d. cos x=-1

Answer
sinx = -1

Explanation
The graph below shown is a sine wave (sine graph). 
This  is because; sin0 = 0, sin90 = 1, sin 180 = 0, sin270= -1, and sin360= 0. This can be confirmed in the graph shown. 
The line shown is the line, y = -1. 

The two intersect at sinx = -1