Statement a is correct. The word secularism was not present in the preamble to the constitution at the time of adoption. However, there can be no doubt that Constitution-makers wanted to establish such a state and accordingly Articles 25 to 28 (guaranteeing the fundamental right to freedom of religion) have been included in the constitution.
Statement b is correct.
The Indian Constitution embodies the positive concept of secularism i.e., all religions in our country (irrespective of their strength) have the same status and support from the state.
Statement c is correct.
The main difference between the American and Indian secularism is that in American secularism, the state does not intervene in religious affairs whatever the situation may arise, but in the Indian secularism, the state can intervene and it often does. For example, Triple Talaq issue, Ram Janmabhoomi Case, Sabarimala cases etc. in which the State intervened and settled the issues amicably.
Statement d is incorrect: The term ‘secular’ was added in the constitution by the 42nd Constitutional Amendment Act of 1976.