A heater boils $x$ amount of water in time $t_1$ and another heater boils the same water in time $t_2$ having resistances $R_1$ and $R_2$ respectively. The ratio $\frac{t_1}{t_2}$ will be:

$\left(\frac{R_1}{R_2}\right)^2$ $\left(\frac{R_2}{R_1}\right)^2$ $\frac{R_2}{R_1}$ $\frac{R_1}{R_2}$

$\frac{R_1}{R_2}$

Since amount of water is $ H = \frac{V^2}{R_1} t_1 = \frac{V^2}{R_2} t_2$ $ \frac{t_1}{t_2} = \frac{R_1}{R_2}$