His boss ………. an explanation of his conduct with his colleagues.

The correct answer is: C. called for

To “call for” something means to demand or request it. In this case, the boss is demanding an explanation of the employee’s conduct.

The other options are incorrect because they do not convey the same meaning.

• To “call up” someone means to telephone them.
• To “call upon” someone means to ask them to do something.
• To “call off” something means to cancel it.