Hello ! posted picture of question

This is not the graph of a linear function. The graph of a linear function is a right line. Then it's not option A. 
This is the graph of a function, because it passes the vertical line test. Any vertical line that you draw only cuts the function in one point. Then it's not option C.
It's not a one-to-one function because it doesn't pass the horizontal line test. There are horizontal lines that cut the graph in more than one point. Then  it's not option D.

Option B. This is the graph of a function, but it is not one-to-one