Which ones is the LCD (Least Common Denominator) out of these three number: 3, 4, and 20?

The least common denominator is 1.

I learnt it as lowest common denominator, and the question needs to be in the form, "which of these three numbers (3,4,20) is the lowest common denominator of * insert number here*"

Need to know which number you are asking the LCD for?