Bulb C is the brightest and draws the most current. If Bulb A is unscrewed, light B will go out but light C will remain at the same brightness. If Bulb C is unscrewed, lights A and B will be unaffected, and remain at the same brightness.
Work Step by Step
Light C carries the most current because the voltage across it is the battery's entire voltage. It burns the brightest. Bulbs A and B share the battery voltage. For two or more devices connected in series, the supply voltage equals the SUM of the voltage drops across the devices. Each identical bulb "feels" only half the voltage of the battery and they do not glow as brightly as bulb C. If bulb A is unscrewed, it interrupts the current in the top branch. Bulb C is still hooked across the battery, and glows the same as before. If bulb C is unscrewed, it strops glowing. The top branch, containing bulbs A and B, is still hooked across the battery, and the 2 bulbs glow as before.