Hey there!
Let's think of what 1000x means.
We know that multiplication is repeated addition. For example, 5(5) = 5+5+5+5+5. It's 5 added five times.
It's no different here. That means we have x 1000 times. If we look at a simpler situation:
5x - 3x, we have (x + x + x + x + x) - (x + x + x)
That means we're getting rid of 3 x's, which means we have 2x. If we have different coefficients but the same variable, we can just subtract or add the coefficients but keep the variable. That means:
4x - 2x = 2x and 10x - 5x = 5x
Now, we can do this problem. We want:
(1000-1)x = 999x
Your answer is D.
Hope this helps!