Adding a resistor in series increases total resistance; adding one in parallel decreases it. Why does adding a component ever make a circuit conduct more easily?

D. Because a parallel resistor adds another route for current.

The pipe analogy is exact enough to be useful: two in series make one longer obstructed route, and two in parallel give the water two routes, so more flows for the same pressure. Series resistances add and parallel conductances add. It is why household wiring is parallel, every appliance gets full voltage and switches independently, and why adding appliances is what a breaker watches for.