## Core Concept
The question pertains to the biphasic action potential seen in mixed nerves. A mixed nerve contains both myelinated and unmyelinated fibers, which contribute to its compound action potential (CAP). The CAP is a summation of action potentials from multiple nerve fibers.

## Why the Correct Answer is Right
The biphasic action potential in mixed nerves is primarily due to the asynchronous activation of the nerve fibers. When a mixed nerve is stimulated, the action potentials generated in the different fibers can produce a biphasic waveform due to the differences in conduction velocities and refractory periods of the fibers. This biphasic action potential is a characteristic feature of the CAP recorded from a mixed nerve.

## Why Each Wrong Option is Incorrect
- **Option A:** This option is not provided, so we cannot assess its accuracy.
- **Option B:** Similarly, this option is not provided for evaluation.
- **Option C:** Without the specific details of option C, we cannot determine why it would be incorrect.
- **Option D:** This option is stated as the correct answer but without context, we can't evaluate the incorrectness of other options directly.

## Clinical Pearl / High-Yield Fact
A key point to remember is that the compound action potential (CAP) of a nerve can exhibit different waveforms (monophasic, biphasic, or triphasic) depending on the recording conditions and the type of nerve being stimulated. The biphasic action potential is commonly observed in mixed nerves due to the mixed population of fibers.

## Correct Answer: D.