When group members withhold their reservations and criticisms for the sake of consensus, they are engaging in: a. expressive leadership b. group solidarity c. laissez-faire decision making d. groupthink?

This is an example of D. groupthink.
Groupthink refers to a phenomenon where members of a particular group will do anything in order to keep the peace and harmony within that group. So even if they don't agree with the ideas of other members, they won't say anything so as not to disturb the peace.

The  ancient egyptians  who  built the pyramids  may have been  able  to move massive stone blocks across the desert by wetting the sand in front of a contraption  built  to pull the heavy objects