India's population has crossed 1.2 billion in May 2011. Which of the following are the probable reasons for this?
(a) Rapid decline in death rate
(b) Increase in number of people at reproducible age
(c) Decline in maternal mortality rate
(d) Decline in Infant mortality rate

All the options (a), (b), (c), and (d) could be probable reasons for India's population crossing 1.2 billion in May 2011:

(a) Rapid decline in death rate: More people survive into adulthood and old age due to better medical facilities.

(b) Increase in number of people at reproducible age: A larger population at reproductive ages could lead to higher birth rates, contributing to population growth.

(c) Decline in maternal mortality rate: A decline in maternal mortality rate indicates better healthcare for pregnant women, which could result in more successful pregnancies and births, contributing to population growth.

(d) Decline in infant mortality rate: A decline in infant mortality rate indicates better healthcare for infants, leading to more infants surviving into childhood and adulthood, which could contribute to population growth.