Atal Pension Yojana (APY) is a government-backed pension scheme in India that aims to provide social security to the unorganized sector and encourage workers to save for their retirement. The correct answer is option 'D', which states that APY provides social security, encourages voluntary savings, and pays a fixed pension on attaining the age of 60 years.

1. Introduction to Atal Pension Yojana:
Atal Pension Yojana was launched by the Government of India in June 2015. It is administered by the Pension Fund Regulatory and Development Authority (PFRDA) and aims to address the pension needs of workers in the unorganized sector who are not covered by any formal social security schemes.

2. Providing Social Security:
The primary objective of APY is to provide social security to individuals working in the unorganized sector. This sector comprises workers in various occupations such as agriculture, construction, small businesses, and self-employment. These workers often lack access to formal pension schemes and face financial uncertainty in their old age. APY helps them secure a regular income after retirement.

3. Encouraging Voluntary Savings:
APY encourages workers to voluntarily save for their retirement by contributing a small amount regularly. Individuals between the ages of 18 and 40 are eligible to join the scheme. They can choose a monthly contribution amount based on their desired pension amount and age at entry. The contribution amount varies depending on the pension chosen and the age at entry.

4. Fixed Pension:
Under APY, a fixed pension is paid to the subscriber on attaining the age of 60 years. The pension amount depends on the contribution made and the years of contribution. There are five pension slabs ranging from Rs. 1,000 to Rs. 5,000 per month. The amount of pension is guaranteed by the government and is directly credited to the subscriber's bank account.

5. Benefits of APY:
- Social security: APY provides a safety net for individuals in the unorganized sector, ensuring a regular income after retirement.
- Affordable contributions: The scheme offers flexibility in choosing the contribution amount based on individual financial capacity.
- Government co-contribution: The government provides a co-contribution of 50% of the subscriber's contribution or Rs. 1,000 per year, whichever is lower, for a period of 5 years for eligible subscribers.
- Portability: APY is portable across the country, allowing subscribers to continue their contributions and receive the pension even if they move to a different location.

In conclusion, Atal Pension Yojana (APY) is a government pension scheme that provides social security, encourages voluntary savings, and pays a fixed pension to individuals in the unorganized sector upon reaching the age of 60. It aims to address the pension needs of workers who are not covered by formal social security schemes and promote financial inclusion and long-term savings.