The question states the the graph is non-vertical. This mean the graph is either rise of falling. This also means that the value of function is not same at more than one value of x.
Another hint the question gives us is that the graph is a straight line. So this means we have a straight line rising or falling with some angle or a slope. This signifies a linear function.
It would have been a non-function if the line was a vertical one.
It would have been a non-linear function if the line was not straight.
The graph of exponential function is not straight.
Hence the answer to this question is Linear Function
