Question

at which point would the circuit need to be severed to prevent all light bulbs from working? 3 1 or 2 4 3 and 4

Explanation:

Step1: Analyze the circuit

In a circuit, if it is a series - parallel combination (assuming from the diagram), to stop all bulbs, we need to break the main current path. Points 1 and 2 are in the main current - carrying part (before the branches start). If we sever at 1 or 2, the current will not be able to flow through any of the branches (where bulbs are located).

Step2: Check other points

Point 3 is in one branch and point 4 is in another branch. Severing at 3 will only stop the bulbs in its branch, and severing at 4 will only stop the bulbs in its branch. Severing 3 and 4 together is also not correct as there are still other paths (if we consider the main path) for current to flow (but if 1 or 2 is intact).

Answer:

1 or 2