Final answer:
The primary limiting factor on the speed of a parallel adder is the number of stages in the adder, due to the cumulative carry propagation delay that occurs as the carry ripples through each stage during an addition operation.
Step-by-step explanation:
The speed of a parallel adder is primarily constrained by the number of stages in the adder. Each stage corresponds to one bit of the operands being added. In a parallel adder, all bits are processed simultaneously, but the carry output from each stage must be available before the next stage can complete its addition. This delay, known as the carry propagation delay, is cumulative and becomes the critical factor that limits the speed of the adder.
Although factors such as clock frequency, power supply voltage, and the number of bits in the adder do impact performance, they are not the primary limiting factors. The clock frequency affects how often a new addition can be started, the power supply voltage impacts the switching speed of transistors, and while having more bits means a larger adder, it doesn't necessarily slow down the operation if the stages can all operate in parallel. It is ultimately the time taken for the carry to ripple through all the stages that limits the maximum speed of a parallel adder.