Answer:
A) f(x) = 1/3x
Explanation:
You can rule out B and C-
B is a quadratic function, seeing as how x is being squared, and C is an absolute value function. Neither of these are linear so you don't have to worry about it here.
That leaves us with A and D, which are both linear.
Linear functions usually take the form of f(x) = mx + b. In a proportional linear function, b = 0.
So with that in mind, you should see that the b value for f(x) = 2x + 11 is 11, which should rule D out as well.
That leaves you with f(x) = 1/3x. As you can see, there's no b value in this function, which means b = 0 and the function is proportional.
Hope this helped :D