Which of the four resistances P, Q, R and S generate the greatest amount of heat when a current flows from A to B ? 

As the terminal PQ is having more resistance than RS, PQ terminal generate less heat than RS terminal as they are connected in parallel. 

$\left[\because \mathrm{Q}=\frac{{\mathrm{V}}^2}{\mathrm{R}}\mathrm{t}\right]$

When we consider R and S that are in series as S > R.

$\because Q=i^{2}Rt$

 S generates more heat than R.