The 42nd Amendment of 1976 inserted the words “socialist” and “secular” — option 4.

What the Preamble said before and after. The original Preamble of 1950 described India as a “SOVEREIGN DEMOCRATIC REPUBLIC”. The 42nd Amendment changed this to “SOVEREIGN SOCIALIST SECULAR DEMOCRATIC REPUBLIC”, and in the same operation altered “unity of the Nation” to “unity and integrity of the Nation”.

The other three options each pair one added word with one original word, so they are half right — which is what makes them effective distractors. Only option 4 names both additions.

Three amendments in one. This is the only occasion on which the Preamble has ever been amended, and the 42nd Amendment is called the “Mini-Constitution” because of its sweep: passed during the Emergency, it made some fifty-nine changes, added the Fundamental Duties in Part IVA, added three words to the Preamble, and extended the life of the Lok Sabha. Much of it was reversed by the 43rd and 44th Amendments after 1977, but the Preamble changes remained.

A point of law. In Kesavananda Bharati (1973) the Supreme Court held that the Preamble is a part of the Constitution and can be amended, but not so as to destroy its basic structure — and secularism has since been held, in S. R. Bommai (1994), to be part of that basic structure. Whether that quality was created by the 1976 insertion or merely made explicit by it is a separate question; the Court has taken the view that the Constitution was secular in substance from the beginning.

Hence, the answer is Secular and socialist.