If you look at the Material variance in a three pillar format, the favourable and adverse result is determined from Material quantity variance and Material price variance. When a manager purchases too much inventory (STD qty), the Quantity variance will be favourable resulting in a total favourable material variance when the price variance is favourable.
The answer A is not correct, because, when a manager purchases too much material, this has the quantitative quality to convert unfavourable price variance to Total material favourable variance. So, quantity is unimportant ; but difference between standard and actual price matters. Hence, price is important for a price variance to show the difference between budgeted values and actual values. Answer C is correct.