**Core Concept:** The Rajya Sabha is the upper house of the Indian Parliament, with its members serving fixed six-year terms. In India, the Constitution mandates the retirement of one-third of the Rajya Sabha members every two years.

**Why the Correct Answer is Right:** Every two years, approximately one-third of the Rajya Sabha members retire due to the mandatory retirement of members upon completion of their six-year term. This ensures a regular rotation of representatives and maintains the composition of the upper house.

**Why Each Wrong Option is Incorrect:**

A. **N/A:** This option is not relevant as it does not address the time frame for member retirements in the Rajya Sabha.
B. **Not Six Years:** This option is incorrect because the correct term length is six years, not a multiple of six.
C. **Twelve Years:** This option is wrong because the retirement occurs every two years, not twelve years.
D. **Not Mandatory:** This option is incorrect because the retirement is mandatory upon completion of the six-year term.

**Clinical Pearl / High-Yield Fact:** Understanding the retirement process of Rajya Sabha members is crucial for medical students aspiring to work in India's healthcare system or related fields, as it impacts political dynamics and public policy decisions.

**Correct Answer:** .