Answer: Human factor research
Step-by-step explanation:
From the question, we are informed that a manager of a bank branch is concerned about the number of mistakes the tellers were making, so he started manipulating different aspects of the environment in the bank to see what effect each has on the tellers' performance.
We are further told that he examined factors such as the lighting, temperature, and the volume of the music playing in the bank.
He's using the human factor research approach. Here, the capabilities of human beings are determined through strength, vision, flexibility etc.