Introduction:
The National Rural Employment Guarantee Act (NREGA) 2005 is a social welfare scheme implemented by the Indian government to provide employment opportunities to rural households and improve their livelihoods. One of the key provisions of NREGA 2005 is the guarantee of a certain number of days of employment to eligible individuals.

Explanation:
The correct answer to the question is option 'A' - 100 days. Here is an explanation of why 100 days is the correct answer:

1. NREGA 2005:
- NREGA 2005 was enacted to enhance the livelihood security of people in rural areas by guaranteeing them employment opportunities.
- It ensures the right to work and mandates the provision of at least 100 days of employment in a financial year to every household whose adult members volunteer to do unskilled manual work.

2. Employment Guarantee:
- The Act guarantees employment to at least one adult member of each household in rural areas.
- The employment provided under NREGA is in the form of unskilled manual work, such as construction of roads, irrigation canals, water harvesting structures, etc.

3. Financial Year:
- The employment guarantee provided under NREGA is calculated on a yearly basis, starting from April 1st to March 31st of the following year.
- Each eligible household can avail a maximum of 100 days of employment in a financial year.

4. Voluntary Participation:
- The Act specifies that the adult members of a household must volunteer to do unskilled manual work in order to avail employment under NREGA.
- The participation is voluntary, and households can choose to avail employment as per their preference and need.

5. Implementation:
- The implementation of NREGA is the responsibility of the state governments, and they are required to ensure the availability of work and timely payment of wages.
- The Central Government provides funds to the state governments for the implementation of the scheme.

Conclusion:
The National Rural Employment Guarantee Act (NREGA) 2005 guarantees at least 100 days of employment in a financial year to eligible households in rural areas. This provision aims to provide livelihood security and improve the socio-economic conditions of rural households. The Act ensures the right to work and emphasizes the voluntary participation of adult members in unskilled manual work. The implementation of NREGA is the responsibility of the state governments, who receive funds from the Central Government for its effective implementation.