I believe it's C. The wagon does not move move because the force you apply to the wagon is equal to the force it applies to you. If the maximum friction force is greater than that force the wagon will not move.
Density = (mass) / (volume)
Density = (35.7 g) / (17 cm³)
Density = 2.1 g/cm³
The minimum power is 490 W
Explanation:
The work that needs to be done by the robot in order to climb the shelves is equal to the gain in gravitational potential energy of the robot + object, therefore:

where
m = 15 kg + 5 kg = 20 kg is the total mass of the system
is the acceleration of gravity
is the change in height
Substituting,

Now we can calcualte the power (wattage) that the robot must have, using the equation

where
W = 3,920 J is the work done
t = 8 s is the time interval
Substituting,

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