The diver has the least gravitational potential energy

. . . when he has gone as low as he can go, and he is at the bottom.

Gravitational potential energy is directly proportional to the height above something or somewhere.

We have the formula for gravitational potential energy is,

E=-GMm/r

where, G is universal gravitational constant

M is the mass of earth

m is the mass of diver

r is the distance between diver and centre of earth

and negative sign(-) is used because work has to be done against the force of gravity.

At constant GMm, E is inversely proportional to the -r.

So when diver goes lower and lower, value of r decreases and makes the magnitude of E high. But the negative sign converts the value into least.

Therefore, diver has least gravitational potential energy.

iron would move it is magnetic there is a mag field around the current in the wire