**Core Concept**
The question is about the optimal age for tendon transfer surgery in patients with post-polio residual paralysis, which is a condition resulting from **poliomyelitis**. This surgical procedure aims to restore muscle function and improve mobility. The ideal timing for such interventions is crucial for maximizing outcomes.

**Why the Correct Answer is Right**
The correct answer is based on the principle that tendon transfer should be performed when the child's bones have reached a certain level of maturity, but before the deformities become too severe or fixed. This typically occurs around the age of 4-6 years for some procedures but can vary based on the specific needs and conditions of the patient.

**Why Each Wrong Option is Incorrect**
**Option A:** This option might be too early, as the bones and muscles may not have developed sufficiently for the procedure.
**Option B:**
**Option D:**

**Clinical Pearl / High-Yield Fact**
A key point to remember is that the goal of tendon transfer in post-polio residual paralysis is to improve function and prevent further deformity. Timing is critical, as it affects the likelihood of successful rehabilitation and the patient's quality of life.

**Correct Answer:** C. 5 years