Final answer:
The correct answer to the fill-in-the-blank question is 'working,' where the manager's continuous action leads to the staff’s exhaustion.
Step-by-step explanation:
The correct answer to the question The manager kept on …………until all of the staff felt exhausted is working. This is because the sentence is describing an action that is continuing over a period of time, and the gerund form of the verb (working) is needed. In English grammar, a gerund is a verb ending in -ing that functions as a noun, and it is often used after the phrase 'kept on' to describe a continuous action.
Therefore, the completed sentence should read: 'The manager kept on working until all of the staff felt exhausted.' This implies that the manager's action of working was ongoing, contributing to the staff's exhaustion.