**Core Concept**
The cremaster muscle is a complex fascial layer that surrounds the spermatic cord, playing a crucial role in the regulation of testicular temperature. Its innervation is essential for the cremasteric reflex, which is a critical indicator of testicular function.

**Why the Correct Answer is Right**
The cremaster muscle receives its motor innervation from the **genitofemoral nerve**, a mixed nerve that arises from the lumbar plexus (L1-L2 spinal nerves). The genitofemoral nerve splits into two branches: the genital branch, which innervates the cremaster muscle, and the femoral branch, which supplies the skin of the anterior thigh. The cremasteric reflex, elicited by stroking the skin of the upper medial thigh, results in elevation of the testis due to contraction of the cremaster muscle.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect as the cremaster muscle does not receive innervation from the obturator nerve, which primarily supplies the adductor muscles of the thigh.
**Option B:** This option is incorrect as the ilioinguinal nerve primarily supplies the skin of the scrotum and the cremasteric muscle is not its target.
**Option C:** This option is incorrect as the iliohypogastric nerve supplies the skin of the lower abdominal wall and the hip, not the cremaster muscle.

**Clinical Pearl / High-Yield Fact**
The cremasteric reflex is an essential clinical sign in the examination of the male genitalia. Its absence may indicate testicular abnormalities, such as torsion or atrophy.

**Correct Answer: D. Genitofemoral nerve.