**Core Concept**
The knee-jerk reflex, also known as the patellar reflex, is a **monosynaptic stretch reflex** that tests the integrity of the **L2-L4** spinal cord segments. It involves the **femoral nerve** and the **quadriceps femoris muscle**.

**Why the Correct Answer is Right**
The correct answer is related to the spinal cord segment responsible for the knee-jerk reflex. The reflex involves the **L2-L4** segments, which are responsible for the **femoral nerve** innervation of the **quadriceps femoris muscle**.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because it does not correspond to the spinal cord segments responsible for the knee-jerk reflex.
**Option B:** Incorrect as it is not related to the knee-jerk reflex.
**Option C:** Incorrect because, although close, it is not the precise root value for the knee-jerk reflex.
**Option D:** Incorrect as it is unrelated to the knee-jerk reflex.

**Clinical Pearl / High-Yield Fact**
The knee-jerk reflex is an important clinical tool for assessing the integrity of the **L2-L4** spinal cord segments and the **femoral nerve**.

**Correct Answer:** L2-L4.