Answer:
A
Step-by-step explanation:
We are given the function f and its derivative, given by: 

Remember that f(x) is decreasing when f'(x) < 0. 
And f(x) is increasing when f'(x) > 0. 
Firstly, determining our zeros for f'(x), we see that: 

Since a is a (non-zero) positive constant, -a is negative. 
We can create the following number line: 
<-----(-a)-----0-----(a)----->
Next, we will test values to the left of -a by using (-a - 1). So:

Since a is a positive constant, (2a + 1) will be positive as well. 
So, since f'(x) > 0 for x < -a, f(x) increases for all x < -a. 
To test values between -a and a, we can use 0. Hence: 

This will always be negative. 
So, since f'(x) < 0 for -a < x < a, f(x) decreases for all -a < x < a. 
Lasting, we can test all values greater than a by using (a + 1). So: 

Again, since a > 0, (2a + 1) will always be positive. 
So, since f'(x) > 0 for x > a, f(x) increases for all x > a. 
The answer choices ask for the domain for which f(x) is decreasing. 
f(x) is decreasing for -a < x < a since f'(x) < 0 for -a < x < a. 
So, the correct answer is A.