**Core Concept**
The cremaster muscle is a thin, fan-shaped muscle in the spermatic cord that regulates the temperature of the testes. It is a key component of the cremasteric reflex, which is essential for testicular function and development.

**Why the Correct Answer is Right**
The cremaster muscle is innervated by the ilioinguinal nerve, which arises from the L1 spinal nerve. This nerve provides motor innervation to the cremaster muscle, enabling it to contract and relax in response to changes in temperature. The ilioinguinal nerve also provides sensory innervation to the skin of the upper medial thigh and the root of the penis. The cremasteric reflex is mediated by the afferent fibers of the ilioinguinal nerve, which stimulate the efferent fibers to contract the cremaster muscle.

**Why Each Wrong Option is Incorrect**
* **Option A:** The genitofemoral nerve is a major nerve that innervates the cremaster muscle, but it is not the primary nerve responsible for its innervation. The genitofemoral nerve arises from the L1 and L2 spinal nerves.
* **Option B:** The obturator nerve is primarily responsible for innervating the adductor muscles of the thigh and is not directly involved in the innervation of the cremaster muscle.
* **Option D:** The femoral nerve is responsible for innervating the muscles of the anterior thigh and is not directly involved in the innervation of the cremaster muscle.

**Clinical Pearl / High-Yield Fact**
The cremasteric reflex is an important clinical sign that can be used to assess the integrity of the ilioinguinal nerve and the cremaster muscle. The reflex is elicited by gently stroking the skin of the upper medial thigh, which should cause the cremaster muscle to contract and elevate the testes.

**Correct Answer: C. Genitofemoral nerve**