An even function is a function where positive and negative values of x give you the same result.
Where f(2) = f(-2) and f(3) = f(-3)
To formally check it, you need to evaluate f(-x) and see if you get the original function back.
Graphically, an even function is symmetrical over the y-axis.
The only graph here that does all of that is f(x) = |x|
