What is the current in a series circuit that has two resistors (4.0 ohms and 7.5 ohms) and a power source of 9.0 volts?

-- First, we have to decide how to handle the two resistors.

The effective resistance of resistors in series is the sum
of their individual resistances.  That is, they act like a single
resistor, whose resistance is the sum of all of them.

So in this question, the 4.0 ohms and the 7.5 ohms act like a
single resistor of 11.5 ohms.

-- The current in the circuit is

                       (the supply voltage) / (the total resistance)

                   =                (9.0 volts)  /  (11.5 ohms)

                   =                      0.783... Ampere        (rounded)

Ithink the answer would be a