A common denominator is 75.
A common denominator, though not the least, is 7500.
It is any multiple of 75.
It is 75.
The greatest common denominator, like the greatest common multiple, is infinite.
LCD(75, 275) = 825
The common denominator of any two or more whole numbers will always be 1.
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.