Explanation

(m−n)(m+n) = 72 with m > n natural. Since m−n and m+n have the same parity and their product 72 is even, both must be even. The even factor pairs of 72 are (2,36), (4,18) and (6,12), giving (m,n) = (19,17), (11,7) and (9,3). No other pairs work, so there are 3 pairs — option A.