12
From the information given,
Wiliam bought 3 pizzas and each had m slices. This means that the total number of slices in the 3 pizzas is 3 * m = 3m
He bought the the pizzas for himslef and 6 friends. This means that the total number of people that shared 3m slices of pizzas is 1 + 6 = 7
Thus, the expression that represent the number of slices of pizza per person are
c) 3m/7