Question

Which function is an even function?

Answer 1

C. cos one because its the only one out of the answer choices that is even in the 12 basic funtions

Answer 2

A function f is an even function, if for all x we have  f(x)=f(-x).

All the functions in the problem are constant functions, because no x is involved.

For example, consider the first function: $f(x)=sin(-3 \pi)$, 

$sin(-3 \pi )$ is just a constant value.( it is actually equal to 0)

Thus for any x, f(x) = 0 and f(-x) =0


This can be said for all constant functions: all of them are even functions.


Answer: all