Hello from MrBillDoesMath!
The first choice (y = 2x) is NOT the blue line because it has twice of the slope of the line y=x. That is, for a given positive x value the graph of y= 2x appears above the black line which is not the case for the blue line shown.
The second choice (y =.001x) is a possibility but the multiplier .001 is so small I think the graph of that line would be "close" to the x-axis. I don't think it's the blue line.
y = -x. Now here we can say NO! That line goes through the origin all right but has a slope of -1 so is heading "downward" from left to right. That is not the blue line either.
Well, then, it seems that the last equation ( y = 1/2x) is the only remaining reasonable answer.
Regards, MrB.