**Core Concept**
The patient's presentation is suggestive of a lumbar disc herniation, specifically a sciatic nerve irritation due to a herniated disc at the L5-S1 level, causing radiculopathy. This condition is characterized by the compression of nerve roots, leading to pain, numbness, and weakness in the affected limb.

**Why the Correct Answer is Right**
The correct diagnosis of a herniated disc at the L5-S1 level can be made based on the patient's symptomatology and physical examination. The sciatic nerve, which is formed by the L4-S3 nerve roots, is commonly compressed at the L5-S1 level, resulting in pain and numbness in the right lower limb. The patient's low backache radiating to the right lower limb is a classic presentation of sciatica, which is often associated with a herniated disc.

**Why Each Wrong Option is Incorrect**
**Option A:** L4-L5 disc herniation would typically present with symptoms in the L4 dermatome, such as weakness in the ankle dorsiflexors and numbness in the medial aspect of the foot.
**Option B:** Cauda equina syndrome is a more severe condition that involves compression of the cauda equina, which is a bundle of nerve roots that arise from the lumbar and sacral spinal cord. This condition presents with more widespread symptoms, including bilateral lower limb weakness, saddle anesthesia, and urinary retention.
**Option C:** Lumbar spondylosis is a degenerative condition characterized by the wear and tear of the lumbar spine, leading to facet joint arthropathy and disc degeneration. While it can cause low back pain, it is less likely to cause radiculopathy or sciatica.

**Clinical Pearl / High-Yield Fact**
The sciatic nerve is formed by the L4-S3 nerve roots, but the most common level of compression is at the L5-S1 level. This is because the L5-S1 disc is the largest and most mobile disc in the lumbar spine, making it more susceptible to herniation.

**Correct Answer: D.**