Answer
The other bulbs increase in brightness after the switch is closed.
Work Step by Step
When the switch is closed, bulb C is effectively removed from the circuit because the switch acts as a short circuit. That is, the current would rather pass through the zero-resistance switch than through bulb C.
This lowers the equivalent resistance of the circuit, so the current that the battery supplies will increase. All of the current passes through the other bulbs, so they increase in brightness after the switch is closed.