Which Amendment added the word 'secular' in the Preamble of the Indian…

Concept: The Preamble is part of the Constitution and can be amended under Article 368, subject to the 'basic structure' limit laid down in Kesavananda Bharati v. State of Kerala (1973). Amendments are numbered in the chronological order Parliament passes them, and each amendment's content is fixed by its own enacting Act -- so identifying which words an amendment added means matching the amendment number to what that specific Act actually changed.

Application: The Constitution (Forty-second Amendment) Act, 1976, enacted on the recommendation of the Swaran Singh Committee during the Emergency, amended the Preamble's text: it replaced 'SOVEREIGN DEMOCRATIC REPUBLIC' with 'SOVEREIGN SOCIALIST SECULAR DEMOCRATIC REPUBLIC' and replaced 'Unity of the Nation' with 'Unity and Integrity of the Nation'. This is the ONLY constitutional amendment that has ever altered the Preamble's wording, and it is the source of the word 'secular' in the Preamble.

Cross-check: Checking the neighbouring amendments confirms none of them touched the Preamble:

• 41st Amendment (1976): raised the retirement age of State Public Service Commission members (Article 316) -- unrelated to the Preamble.

• 43rd Amendment (1977): restored the Supreme Court's and High Courts' judicial-review powers over central laws that the 42nd Amendment had curtailed -- Preamble untouched.

• 44th Amendment (1978): reversed several Emergency-era changes (e.g., property rights) but left the Preamble's 1976 wording as-is.

Since only the 42nd Amendment Act, 1976 changed the Preamble's words, it is the amendment that added 'secular'.