Key Concept: the least common multiple (LCM) of x and y is a multiple of x and a multiple of y.

Let's examine a few multiples:
Multiples of 7: 7, 14, 21, 28, 35,...
Multiples of 10: 10, 20, 30, 40, 50, 60, ,...
Multiples of 3: 3, 6, 9, 12, 15, 18,,...
Notice that the multiples of N are always greater than or equal to N
So, the LCM of x and y must be greater than or equal to x AND greater than or equal to y

Answer choice D (x - y), suggests that x-y is a multiple of x and y
HOWEVER, we can see that x-y is clearly LESS THAN x
As such, x-y cannot be a multiple of x, which also means x-y cannot be the LCM of x and y

Answer: D

Cheers,
Brent