Final answer:
An unfavorable labor efficiency variance most often occurs due to increased time spent on production caused by issues such as machinery problems. The correct option indicating this cause is 'The maintenance of machinery has been inadequate for the last few months.' The other options are less likely to result in an unfavorable efficiency variance. Option c is the correct answer.
Step-by-step explanation:
An unfavorable labor efficiency variance implies that there was more labor time spent on production than was expected at standard rates. Among the options provided, the most likely cause of an unfavorable labor efficiency variance would be inadequate maintenance of machinery. This can cause machinery malfunctions or slowdowns, which in turn require workers to spend more time producing the same amount of goods, thus leading to inefficiency.
Option (a), using highly skilled workers, would generally lead to a favorable efficiency variance because skilled workers tend to work more efficiently. Option (b), increasing wages due to a new labor contract, would not cause an efficiency variance but rather a rate variance. Lastly, Option (d), an improvement in the quality of raw materials, would also typically lead to a favorable variance as it would ease the production process.
Therefore, the correct option answer for the cause of the unfavorable labor efficiency variance is (c) The maintenance of machinery has been inadequate for the last few months.