A common denominator is 75.
LCD (lowest common denominator) refers to fractions and your question did not include any fractions. However, assuming that you're looking to reduce 75/275, the answer is 3/11.
LCM(275, 75) = 825
There is no such thing as a most common denominator.
A common denominator, though not the least, is 7500.
It is any multiple of 75.
825
It is 75.
The greatest common denominator, like the greatest common multiple, is infinite.
525
525
The common denominator of any two or more whole numbers will always be 1.