Which best describes the function notation f(3)=15 a) the output is 3, input is 15 b) output 15 ,input 3 c) output f(x), input 3 d) output f(15), input 3

The function is such that there is an independent variable and dependent variable. Usually, these are represented by x and y. The input is the independent variable, x and the output is the dependent variable, y. 
The input to the function, in this case, is 3 and the output is 15. Hence, the answer to this item is letter B.