Odd functions have the property that the average on the interval [-a,a] is zero, because of this:
The definition of the average of a differentiable function is:
Average = { ∫ f(x)dx from - a to a } / [ a - (-a)]
And for an odd fuction ∫f(x)dx from - a to a is zero => Average = 0
The only odd function in the list is cos(x), then the answer is b. cos(x).
