Answer:
b. the sum of the voltages across each bulb.
Step-by-step explanation:
As we know that the total voltage across the given series circuit will give some current to the circuit
The current will remain the same in the circuit in all the resistors.
So here we will have voltage across each resistor given by



and so on
so here for total voltage across all the resistors we can say

so correct answer will be
b. the sum of the voltages across each bulb.