Which three relationships are true at equilibrium? ecell = 0, and ∆g = 0, and q = k ecell > 0, and ∆g < 0, and q = k ecell < 0, and ∆g < 0, and q < k ecell = 0, and ∆g > 0, and q = k ecell = 0, and ∆g > 0, and q > k

The correct answer is option A. i.e. Ecell = 0, and ∆G = 0, and Q = K

Ecell is the cell potential or voltage between the two cells.
∆G is the Gibbs free energy. It is actually a difference in the energies of products and reactants.
K is the equilibrium constant for any reaction.
As you can see from the figure attached, the conditions Ecell = 0, and ∆G = 0, and Q = K favour no reaction, which means, the equilibrium is maintained.

The net force is 100

Ican't see the chart but the density of both nickel and cobalt is 8.90 g/cm3, so it would be one of those.