Final answer:
An unfavorable material quantity variance can be caused by purchasing substandard material, leading to greater waste or inefficiencies during production.
Step-by-step explanation:
The possible cause of an unfavorable material quantity variance is purchasing substandard material. This can occur when the materials bought are of lower quality or not up to the specifications required for production, leading to increased waste or inefficiencies in the manufacturing process. Therefore, if more material than expected is used to produce a set number of units, the variance would be considered unfavorable.