**Core Concept**
The question is testing the knowledge of muscles responsible for movements of the wrist joint. The wrist joint is a complex structure that enables flexion, extension, abduction, adduction, rotation, and circumduction. The muscles involved in wrist movements are primarily the flexor and extensor muscles of the forearm.

**Why the Correct Answer is Right**
The adduction of the wrist joint is primarily caused by the **flexor carpi ulnaris** muscle. This muscle is one of the two muscles responsible for adduction of the wrist joint. The flexor carpi ulnaris muscle originates from the medial epicondyle of the humerus and inserts into the pisiform bone, the hook of the hamate, and the base of the fifth metacarpal bone. It works by contracting and pulling the wrist medially, thus causing adduction.

**Why Each Wrong Option is Incorrect**
**Option A:** The flexor carpi radialis is primarily responsible for wrist flexion and radial deviation, not adduction.
**Option B:** The extensor carpi radialis brevis is primarily responsible for wrist extension and radial deviation, not adduction.
**Option C:** The palmaris longus is primarily responsible for wrist flexion, not adduction.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the flexor carpi ulnaris muscle is also responsible for ulnar deviation of the wrist, which is often seen in conditions such as ulnar nerve palsy.

**Correct Answer:** D.