Answer
Voltmeter readings, greatest to least: A, B, C.
Work Step by Step
For two or more devices connected in series, the supply voltage equals the sum of the voltage drops across the devices.
This is Rule 4 for series circuits, on page 443, and discussed on pages 442-443.
In case A, the voltage across the one bulb is the same as the battery voltage.
In case B, the bulbs are identical, and wired symmetrically. The voltage across each bulb is one-half the battery voltage, and the voltmeter reading is one-half of what it is in case A.
In case C, the bulbs are identical, and wired symmetrically. The voltage across each bulb is one-third the battery voltage, and the voltmeter reading is one-third of what it is in case A.
