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Answer (D) is correct . Mix and yield variances are the components of the usage (quantity or efficiency) variance. Mix and yield variances can only be calculated when inputs are substitutable. The yield variance measures how efficiently the actual mix of inputs was used to produce the given output. The yield variance equals standard total quantity (STQ) minus the actual total quantity (ATQ) times the weighted-average standard price of the standard mix of inputs (SPSM).

Answer (A) is incorrect because The weighted-average expected price for inputs is used in calculating the mix variance but not the yield variance.
Answer (B) is incorrect because This is the formula for the mix variance.
Answer (C) is incorrect because The weighted-average actual price for inputs is not used in calculating the mix and yield variances.