LCM stands for 'Lowest Common Multiple' which basically means "what's the smallest number that has the numbers in its multiplication table, excluding 1."*
So, the only numbers that are LCMs of 16 and 20 are 2 and 4. The lowest one is 2, so that's your answer.
* It's actually a little more complex than that, but I guess that was the easiest way to put it.
