The question pertains to a specific `Population by Religion` group in India based on the values provided for different decades from the Census data. It doesn`t specify any particular details about each option`s population numbers, so we have to go with the option that makes the most sense.

Option 1: Sikhs - This is not the answer because the Sikh population in India is considerably higher than the given data.

Option 2: Jains - This is also incorrect. The Jain population is much smaller compared to the figures mentioned in the question.

Option 3: Christians - This is the correct answer. The increasing trend in numbers and the values align most closely with the Christian population as per the Indian Census data.

Option 4: Buddhists - This is incorrect. The Buddhist population in India is less compared to the statistics provided in the question.

Without having access to the actual Census data for all these religious groups` population, we can`t provide a more concrete explanation for the choice. But, based on the information given, the correct answer is most likely Christians.