The option that may help explain why a firm is experiencing increasing marginal returns is:
More efficient workers
Increasing marginal returns occur when adding additional units of input (such as labor) results in a greater increase in output. This can happen if the current workers are becoming more efficient due to better management, improved processes, or teamwork, allowing them to produce more output with each additional worker.
The other options—poorly trained workers, overcrowded workers, and lack of specialization—are more likely to result in diminishing marginal returns, where additional input leads to smaller increases in output or even a decrease in output.