Remark
A function is defined as 1 y value per x value. The y values can be the same for 2 x values, but the x values must have just 1 value associated with it. I am describing a vertical line test for a function. This is a function.
A 1 to 1 function means that the function obeys the horizontal line test. That means that for every x there is only 1 y value.
A
This graph is both a function and a 1 to 1 function. So the answer is A.
None of the other answers are this complete and one of them is wrong.
B
B is wrong. It is a 1 to 1 function.
C
C is wrong. The graph is a function.
D
D is wrong. No y values are duplicated.