If the marginal product of labour starts falling, this means that _____.

the firm is operating at optimal capacity
task specialization is no longer economically viable
total output is increasing at an increasing rate
the firm’s variable costs are too high

Explanation
Task specialization helps to raise productivity, as it results in increasing marginal product for additional workers, but after a certain point, it diminishes. There is only so much task specialization that can occur without leaving a worker without a full day’s work.