Moderate# Mersenne Number GCD and LCM

NUMTH-SBJWNS

Mersenne number is a positive integer of the form $M_n:=2^n-1$ for a positive integer $n$.

We write $(m,n)$ for the greatest common divisor of $m$ and $n$, and $[m,n]$ for the least common multiple of $m$ and $n$.

What is the least common multiple of $M_m$ and $M_n$? Select **ONE** answer which is always true.