Answer (C) is correct . The materials usage variance is typically influenced most by activities within the production department. It is the variance most controllable by a production manager.
Answer (A) is incorrect because Overhead variances are by definition affected by activities outside the production area.
Answer (B) is incorrect because The efficiency of employees affects the labor efficiency variance.
Answer (D) is incorrect because The overhead volume variance measures the effect of not operating at the budgeted activity level. It is the least controllable by a production manager.
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