Why is the answer B?

I assumed that its not drawn to scale (the slope may not be 1 as the drawing may suggest), so it could be that in reality the line is fully vertical in which case x = 0 ( x=0, y=5). Why is this line of thinking wrong?

Firstly, coordinate geometry IS to scale in the GRE I believe.

If it looks like point R is not on the axes, you CAN safely assume its not on the X or Y axes, so it cannot be a vertical line.