**Core Concept**
The question is testing the understanding of legal aspects of mental retardation, specifically the age of marriage. In India, the Special Marriage Act of 1954 and the Hindu Marriage Act of 1955 have provisions related to the age of marriage for individuals with mental retardation.

**Why the Correct Answer is Right**
The law in India allows for the marriage of a mentally retarded person with a guardian's consent, provided the person is at least 21 years of age. This is in accordance with the Mental Health Act of 1987, which empowers the guardian to make decisions on behalf of the individual. The guardian's consent is essential to ensure the person's well-being and to prevent exploitation.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it is not a specific age mentioned in the legal provisions related to mental retardation.
**Option B:** This option is incorrect because it is below the minimum age of 21 years, which is the legal requirement for marriage of a mentally retarded person with a guardian's consent.
**Option D:** This option is incorrect because it is not a specific age mentioned in the legal provisions related to mental retardation.

**Clinical Pearl / High-Yield Fact**
It is essential for doctors and healthcare professionals to be aware of the legal aspects of mental retardation, including the age of marriage, to provide comprehensive care and support to individuals with mental retardation.

**Correct Answer: B. 18 years**