Assertion: Renal cells are involved in stimulating the formation of RBCs.
Reason: The juxtaglomerular cells of kidney produce erythropoietin.

Answer

To solve the question regarding the assertion and reason, we will analyze both statements step by step.

### Step 1: Understand the Assertion
The assertion states that "Renal cells are involved in stimulating the formation of RBCs."
- Renal cells, specifically the juxtaglomerular cells in the kidneys, play a role in the regulation of red blood cell (RBC) production.

### Step 2: Understand the Reason
The reason states that "The juxtaglomerular cells of the kidney produce erythropoietin."
- Juxtaglomerular cells are specialized cells in the kidneys that detect low oxygen levels in the blood. In response, they produce and release erythropoietin (EPO), a hormone that stimulates the production of RBCs in the bone marrow.

### Step 3: Connect the Assertion and Reason
- The assertion is correct because renal cells (juxtaglomerular cells) are indeed involved in stimulating RBC formation.
- The reason is also correct because the juxtaglomerular cells produce erythropoietin, which is responsible for stimulating the production of RBCs.

### Step 4: Determine the Relationship
- The reason provided explains how the assertion is true. The production of erythropoietin by juxtaglomerular cells leads to the stimulation of RBC formation.

### Conclusion
Since both the assertion and the reason are true, and the reason correctly explains the assertion, the answer is:
**Option A: Both assertion and reason are true and reason is correct explanation for assertion.**

---