Final answer:
Adding a fourth bulb in parallel with bulb B does not affect the brightness of bulb A, and ideally keeps the brightness of bulbs B and D unchanged as well, as each is connected directly to the voltage source.
Step-by-step explanation:
When a fourth bulb (D) is added in parallel with bulb B, but not in parallel with B and C, the overall resistance of the circuit decreases because parallel connections provide multiple pathways for the electric current. In parallel circuits, each component is connected directly to the voltage source; therefore, each bulb will continue to receive the same voltage it did before the additional bulb was added.
Bulb A is unaffected by the change since it is not in the parallel branch with bulb B and the new bulb D. The brightness of bulb A remains constant. Bulbs B and D will each draw current as if they were the only bulb connected to the voltage source. However, since the total current in the circuit increases with the addition of bulb D, bulbs B and D may have a minor effect on each other depending on the internal resistance of the power source. If the source can provide enough current without a significant drop in voltage, the brightness of all bulbs should ideally remain the same. If there is a voltage drop due to internal resistance or current limits of the source, the brightness of the bulbs may slightly decrease.