Answer:
C.
Step-by-step explanation:
p(x)=sin(x) is an odd function since sin(-x)=-sin(x).
q(x)=cos(x) is an even function since cos(-x)=cos(x).
r(x)=tan(x) is an odd function since tan(-x)=-tan(x).
s(x)=csc(x) is an odd function since csc(-x)=-csc(x).
So the only contender seems to be C.
Let's check. To check we have to plug in (-x) in place of (x) and see if we get the same function back since we are looking for an even function.
[tex]f(x)=\cos(\frac{5\pi}{4}x)[/tex]
Replace (x) with (-x):
[tex]f(-x)=\cos(\frac{5\pi}{4}(-x)[/tex]
[tex]f(-x)=\cos(\frac{-5\pi}{4}x)[/tex]
[tex]f(x)=\cos(\frac{5\pi}{4}x)[/tex] since cosine is even; that is cos(-u)=cos(u) where u in this case is [tex]\frac{5\pi}{4}x[/tex].
So f is even.
