Which of the following is the parent function of all absolute value functions? f(x) = 3x f(x) = |x| f(x) = 2|x| f(x)=x^2

Hello, 

The parent function of all absolute value functions is the f(x) = |x|.
I'll explain why. 
f(x) = 3x is a linear function.
f(x) = 2|x| is an absolute value function, but it has a vertical stretch by a factor of 2, and we want to find the parent function, it needs to be in the most simplest form. 
f(x) = x² is a quadratic function.

So, the final answer is choice 2, f(x) = |x|.