Explanation:

Third option is correct.
It is true that Transgender population was counted for the first time. Earlier there were only two genders enumerated in the census data i.e. either male or female.
For the first time a ‘no-religion’ category was introduced and more than 2 million people registered themselves under this category. This is in line with the Article 25 of Indian constitution.
District Thane was most populous and District Dibang Valley of Arunachal.
Any one above age 7 who can read and write in any language with an ability to understand was considered a literate. In censuses before 1991, children below the age 5 were treated as illiterates. The literacy rate taking the entire population into account is termed as "crude literacy rate", and taking the population from age 7 and above into account is termed as "effective literacy rate".
The literacy rate we talk about generally is effective literacy rate.