The correct answer is: D. normal consumption x normal reorder period.
Reorder level is the level of inventory at which a new order should be placed. It is calculated by multiplying the normal consumption rate by the normal reorder period. The normal consumption rate is the average rate at which inventory is used. The normal reorder period is the average time between orders.
The other options are incorrect because they do not take into account the normal consumption rate or the normal reorder period. Option A, maximum level – minimum level, is the difference between the maximum and minimum inventory levels. This is not a useful measure for determining when to reorder, because it does not take into account the rate at which inventory is used. Option B, maximum consumption X maximum reorder period, is the product of the maximum consumption rate and the maximum reorder period. This is also not a useful measure for determining when to reorder, because it does not take into account the average consumption rate or the average reorder period. Option C, minimum consumption x minimum reorder period, is the product of the minimum consumption rate and the minimum reorder period. This is also not a useful measure for determining when to reorder, because it does not take into account the average consumption rate or the average reorder period.