Final answer:
A skillful workforce is most likely to contribute to a favorable variable overhead efficiency variance when machine-hours are used as a cost-allocation base because such workers enhance the efficient use of machinery.
Step-by-step explanation:
When machine-hours are used as a cost-allocation base, the item most likely to contribute to a favorable variable overhead efficiency variance is a skillful workforce. This is because a skillful workforce can operate machinery efficiently, minimizing the time and costs associated with machine-hours. Excessive machine breakdowns, additional machinery, and strengthened demand for the product do not directly contribute to the efficiency of machine-hour usage. Instead, they could lead to increased costs or changes in production volume but do not inherently improve efficiency.
For example, with higher wages incentivizing more investment in machines, the firm will have an incentive to use plans that require fewer labor hours by operating more machinery. Having skillful workers who can utilize the machinery effectively ensures that the labor hours that are used are as productive as possible, potentially resulting in fewer machine-hours needed for production, hence contributing to a favorable efficiency variance.