Answer: In the first case the force decreases proportionally to 1/ε, but in the second case it increases proportionally to ε.
Explanation:
If the charge q remains unchanged, then a change of dielectric leads to a decrease in the field intensity to 1/ε times its former value, i.e., E2 = E/ε. As a result, the force acting on each of the spheres F' = q.E also decreases by the same amount.