Two identical immersion heaters are to be used to heat water in a large container.
Which one of the following
arrangement would heat the water faster, connecting the heaters in series with the main supply,
connecting the heaters in parallel with the main supply ?

Heat produced in a heater, when connected to main supply,
When identical heaters (i.e. having same resistance) are connected in parallel,
their net resistance decreases as compared to when connected in series.
Therefore, heaters connected in parallel would heat the water faster as
the heat produced in parallel combination is more than the heat produced in series combination,
on same electric energy and electric power.