Answer:
Only A 
Explanation:
Let's remember the definition of a function: 
A function is a relation from a set of inputs to a set of possible outputs where each input is related to exactly one output. 
 In A, each input has exactly one output so it's a function.
But in B, the inputs 4 and 9 have two different outputs, it can't represent a function. 
The answer is only A.