**Core Concept**
The Bennett fracture involves the base of the first metacarpal bone, which is a critical anatomical structure for thumb movement. The intrinsic muscles of the thumb play a crucial role in its function, including opposition, abduction, and flexion.

**Why the Correct Answer is Right**
The primary intrinsic muscle responsible for thumb opposition, which is impaired in a Bennett fracture, is the opponens pollicis. This muscle originates from the trapezium bone and inserts into the proximal phalanx of the thumb. It plays a vital role in opposition, which is the ability to move the thumb towards the palmar surface of the other fingers. The opponens pollicis muscle is innervated by the recurrent branch of the median nerve.

**Why Each Wrong Option is Incorrect**
* **Option A:** The abductor pollicis brevis muscle primarily aids in thumb abduction, which is not the primary function impaired in a Bennett fracture.
* **Option B:** The flexor pollicis brevis muscle is responsible for thumb flexion, but it is not the primary muscle responsible for opposition.
* **Option D:** The adductor pollicis muscle is responsible for thumb adduction, which is not directly related to the opposition function impaired in a Bennett fracture.

**Clinical Pearl / High-Yield Fact**
Intrinsic muscles of the thumb, particularly the opponens pollicis, are susceptible to injury in fractures involving the base of the first metacarpal bone, such as Bennett fractures.

**Correct Answer:** C.