For this case, what you must take into account when storing materials is the weight of the materials.
It is convenient that we can store the materials at a level where the back does not have much effort when loading the materials.
At the level of eye, the effort is high because we must move the arms upwards and then carry the weight to the waist.
At ground level the effort is high because we have to tilt our backs to pick up the weight.
The ideal level is the waist because we should not tilt the back, therefore, we make less effort.
Answer:
The best way to prevent back injury is to store materials at waist level.
option c.