The answer to your problem is A due to the slope of your line being -1/3x.
You can find this slope by picking two points on your graph [i.e. (-1,3) and (0,0)].
Find the difference between the two points, which is a one for the x value and a three for the y values. Now you have a slope of 1/3.
But wait! The slope is downwards, therefore a negative must be applied to your slope.
This provides you with a slope of -1/3x, therefore:
y = -1/3x