The only marked point which is in the circuit branch of the front lamp is D. The switch at D controls the on-off mechanism because an open switch causes the circuit to be “broken” at that point, so there the circuit becomes temporarily incomplete and the lamp will not switch on.

If we place it at B instead (the power source), we control the main power for the entire branch since the circuit leading to the battery will be incomplete. This means that all the lamps will either be switched on at the same time or switched off at the same time.