**Core Concept**
The question is testing the student's knowledge of the types of nerve fibers and their associated conduction velocities. Nerve fibers can be classified into three main types based on their diameter and conduction velocity: A-alpha, A-beta, A-gamma, and A-delta fibers, as well as C and B fibers.

**Why the Correct Answer is Right**
The fastest acting nerve fibers are the A-alpha fibers, which have a conduction velocity of approximately 120 meters per second (m/s). These fibers are primarily responsible for transmitting motor signals from the central nervous system to muscles and also transmit proprioceptive and sensory signals from muscles and joints to the central nervous system. A-alpha fibers have a large diameter, which allows for rapid transmission of signals.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because A-beta fibers have a conduction velocity of approximately 80 m/s, which is slower than A-alpha fibers.
**Option B:** This option is incorrect because A-gamma fibers have a conduction velocity of approximately 25 m/s, which is significantly slower than A-alpha fibers.
**Option C:** This option is incorrect because C fibers have a conduction velocity of approximately 2 m/s, which is much slower than A-alpha fibers.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that the conduction velocity of nerve fibers decreases as their diameter decreases. This is why A-alpha fibers, with their larger diameter, are the fastest acting.

**Correct Answer:** A. A-alpha fibers.