State Lenz’s law.

Two identical aluminium balls are dropped simultaneously from the same height. Ball P falls through a region with no magnetic field. Ball Q falls through a region of uniform horizontal magnetic flux density B.

Explain why ball Q takes longer than ball P to reach the ground.

induced emf/induced current acts so as to oppose the change causing it;

ball Q enters/leaves magnetic field / experiences changing flux;
so an emf/current is induced;
this causes a magnetic field;
which opposes the motion of / exerts an upward force on ball Q;

or in terms of energy:
ball Q moves through a magnetic field / experiences changing flux;
so an emf/current is induced;
current causes dissipative heating due to resistance;
some kinetic energy changes to thermal energy;