**Core Concept**
The Factories Act of 1976 is a legislation aimed at protecting the health, safety, and welfare of workers in factories. It sets various standards and regulations to prevent occupational hazards and ensure a safe working environment. The Act prohibits the employment of children in factories to prevent exploitation and ensure their education and well-being.

**Why the Correct Answer is Right**
The Factories Act of 1976 prohibits the employment of children below the age of 14 years in factories. This is based on the principle of protecting children from exploitation and ensuring they receive education. The Act also sets out provisions for the education and training of children, and prohibits their employment in hazardous occupations. The age of 14 is considered the minimum age for employment in factories as it is believed that children below this age are not physically or mentally mature enough to handle factory work.

**Why Each Wrong Option is Incorrect**
**Option A:** 10 years - This age is too young and does not align with the provisions of the Factories Act, which sets the minimum age for employment at 14 years.
**Option C:** 18 years - While 18 years is considered the age of majority in many countries, it is not the minimum age for employment in factories as per the Factories Act.
**Option D:** 21 years - This age is also not relevant to the Factories Act, which specifically prohibits the employment of children below 14 years in factories.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the Factories Act of 1976 is a crucial legislation that protects the rights and welfare of workers in factories, particularly children. Familiarizing oneself with the provisions of the Act is essential for anyone working in the field of occupational health and safety.

**✓ Correct Answer: B. 14 years**