All of the teachers in the social science department willingly share their ideas and expertise with each other. They are good at ________ communication.

A) upward
B) downward
C) diagonal
D) horizontal

The best choice is horizontal communication because it refers to communication between individuals at the same hierarchical level or within the same department, which is the case with the teachers in the social science department sharing their ideas and expertise with each other. The other choices do not accurately describe the nature of communication between the teachers. Upward communication would refer to communication from subordinates to superiors, while downward communication would refer to communication from superiors to subordinates. Diagonal communication refers to communication between individuals at different levels or in different departments.