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

If you are given a function, say f(x) = mx+c
This function would give you the relation between two variables, in other words it tells you the output when an input "x" is inserted.

The input for your function would be "x" and the output would be the result of the equation given.

Based on this:
for f(3) = 15, the input (x) would be 3 and the output would be 15

Therefore, the correct choice is:
B) the output is 15, the input is 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. 

/-/////////looks tough ‍♂️