Under which Amendment were the words ‘Socialist’ and ‘Secular’ inserted into…

The Preamble is the Constitution's introductory statement of its guiding philosophy and core values. It forms part of the Constitution and can be amended by Parliament under the Article 368 procedure, subject to the basic-structure doctrine laid down in Kesavananda Bharati v. State of Kerala (1973); any wording added to it must trace to a specific numbered Constitutional Amendment Act that enacted the change.

1. As adopted on 26 November 1949 (in force 26 January 1950), the Preamble described India only as a 'SOVEREIGN DEMOCRATIC REPUBLIC' — with no reference to 'Socialist' or 'Secular'.

2. The Constitution (Forty-second Amendment) Act, 1976 — enacted during the Emergency on the recommendation of the Swaran Singh Committee — amended the Preamble's text.

3. It inserted 'SOCIALIST' and 'SECULAR' between 'SOVEREIGN' and 'DEMOCRATIC', and changed 'unity of the Nation' to 'unity and integrity of the Nation'.

4. The amended clause therefore reads: 'SOVEREIGN SOCIALIST SECULAR DEMOCRATIC REPUBLIC'.

5. The same Act also inserted Part IVA (Fundamental Duties, Article 51A), which is why the 42nd Amendment is widely called the 'Mini-Constitution'.

Cross-check: the neighbouring amendment numbers from the same 1975–76 period addressed unrelated subject matter, none of it touching the Preamble text:

Since none of the 39th, 40th, or 41st Amendments touched the Preamble, this confirms the 42nd Amendment as the one that changed it.

Hence the amendment that inserted 'Socialist' and 'Secular' into the Preamble is the 42nd Amendment (1976).