**Core Concept**
Tardy ulnar nerve palsy is a delayed-onset ulnar nerve neuropathy that occurs due to a gradual stretching or compression of the ulnar nerve at the elbow, often secondary to cubitus valgus deformity. This condition typically presents in adolescence or young adulthood, several years after the initial injury.

**Why the Correct Answer is Right**
The correct answer is related to the mechanism of cubitus valgus deformity. Cubitus valgus deformity is a common complication of supracondylar fractures of the humerus, where the distal fragment is displaced in a valgus direction. Over time, the ulnar nerve, which runs close to the medial epicondyle, becomes stretched or compressed due to the gradual displacement of the medial epicondyle, leading to tardy ulnar nerve palsy.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not accurately describe the underlying cause of tardy ulnar nerve palsy. While ulnar nerve compression can occur at various sites, cubitus valgus deformity is a specific and common cause of this condition.
**Option B:** This option is incorrect because it is a general term that does not specifically relate to the cause of tardy ulnar nerve palsy. While ulnar nerve entrapment can occur at various sites, it is not a specific cause of this condition.
**Option C:** This option is incorrect because it does not accurately describe the underlying cause of tardy ulnar nerve palsy. While ulnar nerve compression can occur at various sites, cubitus valgus deformity is a specific and common cause of this condition.

**Clinical Pearl / High-Yield Fact**
A key clinical characteristic of tardy ulnar nerve palsy is the delayed onset, often several years after the initial injury. This is in contrast to acute ulnar nerve injuries, which typically present shortly after the initial trauma.

**Correct Answer: C. Cubitus valgus deformity.**