"Goes downwards from left to right" is basically saying that the slope of this line is negative. We go downhill from left to right.
y = 2x+5 has a positive slope because m = 2 is the slope (b = 5 is the y intercept)
So choice A is out
Similarly choice B is eliminated as well because the slope is also m = 2
Choice C is false as well. The slope here is m = 2 (think of y = 2x as y = 2x+0)
The only choice with a negative slope is choice D. The slope is m = -2
This is why the answer is choice D
Side Note: I'm using slope intercept form y = mx+b where m is the slope and b is the y intercept