Answer:
c. decrease
Step-by-step explanation:
The law of diminishing marginal return confers that adding more of a unit of production while holding the other inputs constant will lead to a decline in productivity. Employing more units of a particular input will increase productivity until the optimal level is achieved. Beyond the optimal level, the benefits from an extra input will increase at a decreasing rate and eventually losses.
Adding more custodians will be beneficial up to the level only. Any extra custodians past the optimal level will cause a reduction in labor productivity.