Correct option is D
3*4 = 21 ; 4*5 = 36 ; 5*6 = 55
Pattern followed is :
For 3*4 = 21,
3 × (3 + 4) = 3 × 7 = 21
For 4*5 = 36,
4 x (4+5) = 4 x 9 = 36
For 5*6 = 55
5 × (5 + 6) = 5 × 11 = 55
Similarly,
For 6*7
6 × (6+7)= 6 × 13 = 78