Answer: a. These two competitor firms will negate each other's efforts.
Step-by-step explanation:
The advertising campaigns that both monopolistic competitors was said to be successful which means that they were both able to draw their competitor's customers away from the other firm.
The net effect of this would be that both of them negated each other's efforts because when Firm A gained some of Firm B's customers it also lost some of its customers to Firm B which is evidently also what happened to Firm B.