The best source of data for the estimation of crude birth rate of India is 

Sample Registration System (SRS): The SRS is a large-scale demographic survey conducted continuously by the Office of the Registrar General of India. It was specifically initiated in the 1960s to provide reliable annual estimates of infant mortality rate, child mortality rate, crude birth rate, and crude death rate at both national and sub-national levels. It combines continuous enumeration with retrospective surveys, making it the most accurate and updated source for annual vital statistics in India.

Why not the others?

(A) National Family Health Survey (NFHS): While highly detailed, the NFHS is a periodic survey (conducted every few years) rather than a continuous tracking system for annual crude rates.

(B) Civil Registration System (CRS): The CRS records births and deaths continuously, but in India, it suffers from under-reporting and incomplete registration in several states, making it less reliable for direct nationwide statistical estimation.

(D) Census of India: The Census provides absolute, comprehensive data but is only conducted once every ten years, meaning it cannot provide regular, up-to-date annual estimations.