Since profit = revenue - cost, or P = R - C, we need to determine a value of p such that revenue > cost. We have that p + 15 = cost and p(9 - p) = revenue. Therefore, the profit is P = p(9 - p) - (p + 15) = 9p - p^2 - p - 15 = -p^2 + 8p - 15.

Notice that the graph of P = -p^2 + 8p - 15 is a parabola that opens downward, so the maximum value is at its vertex. Recall that we can use the formula -b/2a to find the x-value (in this case, the p-value) of the vertex:

p = -b/2a = -8/(2(-1)) = -8/-2 = 4

Answer: B