A. Growth rate started slowing down gradually: During this phase, the population growth rate in India begins to gradually slow down. This is often a result of various factors such as improvements in healthcare, education, and family planning, leading to a decline in the overall fertility rate.

B. There is a downward trend of crude Birth Rate: In phase IV, there is a noticeable decrease in the crude birth rate, which is the number of live births per 1,000 individuals in a population. This decline is largely influenced by changing societal norms, increased awareness about family planning, and the adoption of birth control measures.

C. Period of population explosion in India: is not a characteristic of phase IV, it is often associated with earlier phases of the demographic transition in India. The features of phase IV reflect a transitional period where the population growth rate begins to stabilize, birth rates decline, and societal norms regarding marriage and family planning undergo significant changes.

D. Increase in the mean age at marriage: As part of the demographic transition, there is an observable increase in the mean age at marriage. This change is often associated with societal and cultural shifts, as well as increased educational and career opportunities for women, leading to a preference for later marriages and delayed childbirth.

So, option 1 is correct.