You need at least two denominators to find an LCD. If that's 4 and 5, the LCD is 20.
The greatest common denominator is infinite. The least common denominator is 45.
The least common multiple is 1440
If I wanted to be nasty about it, I would say 1, because the least common denominator of 45/1 and 75/1 is 1. The least common denominator of two fractions is the least common multiple of their denominators. I presume you are asking for the least common multiple of 45 and 75, which is 225.
45
45
360
It is: 45
45
20
LCD(30, 45) = 90
It is 45 which is also their LCM
45