Hi there!
Recall the following relationships:

W = Work (J)
U = Potential Energy (J)
m = mass (kg)
g = acceleration due to gravity (9.8 m/s²)
h = height (m)
t = time (s)
Begin by calculating the work:

Now, divide by the time to solve for power:

**W is the unit for power (Watts). Be careful not to get the two confused.
Answer:
1.02453 Am²
0.36834 m
Explanation:
N = Number of turns = 580
d = Diameter = 2.1 cm
r = Radius = 
A = Area = 
I = Current = 5.1 A
B = Magnetic field = 4.1 μT
x = Distance
= Vacuum permeability = 
Magnetic dipole moment is given by

The magnitude of the magnetic dipole moment of this device is 1.02453 Am²
Magnetic field is given by

The axial distance is 0.36834 m
The answer is B because that that will make it more powerful but less lasting.