#### Answer

$(m+n)(m-n)$

#### Work Step by Step

We are given the denominators $m+n$, $m-n$, and $m^{2}-n^{2}$.
In order to find the least common denominator, we must factor each denominator.
$m+n$ cannot be simplified further
$m-n$ cannot be simplified further
$m^{2}-n^{2}=(m+n)(m-n)$
Next, we multiply together all distinct factors from each denominator, with each factor raised to the greatest power that occurs in any denominator.
$LCD=(m+n)(m-n)$