Article: If you have been living in India for at least 7 years, you will use section 5(1)(a). If you've lived outside of India, you will use section 5(1)(b) Indian descent is defined as a person who was born after August 15, 1947 in India or a territory that became a part of India or if she has a parent that fulfills that qualification. In other words, you can become an Indian citizen because you are legally married to an Indian citizen. You must have been living in India for 7 years. You will use section 5(1)(c). To use this form, your parents must be registered as citizens, not just born in India. This form can apply to minor children or adult children.  Parents of minors applying for citizenship should use the plain Form-III under section 5(1)(d). If your parents are registered as citizens under 5(1)(a) or 6(1), you should use Form-III-A under section 5(1)(e).  If your parents were previous residents of Independent India and you have been in India a year prior to applying, you will use Form-III-B and file under section 5(1)(f). You can become a citizen of Indian by naturalization if you've lived in the country for at least 12 years. You make this application under section 6.  You may not have been living in India illegally.  You may be able to file for citizenship earlier if you've made a significant contribution to the world, especially in literature, art, philosophy, or science.

What is a summary?
Use Form-I if you are of Indian descent. Use Form-II if you want to gain citizenship by marriage. Fill in Form-III if you are a child of Indian parents. Apply with Form-XII for naturalization.