Which of the following increases the tendency for groupthink? a. group is socially engaging with nonmembers often b. group has not chosen a respected leader c. group is under pressure to make a decision d. group lacks cohesiveness

The one that increases the tendency for groupthink: C. group is under pressure to make a decision 

A groupthink happen when ALL members of the group have the tendency to conform with one another.
When the group is under pressure to make a decision, most people wouldn't want to held responsible by making a wrong decision. So most people will prefer to hear the first person that make an idea and follow it to let that person took the fall.

Your answer is going to be c.