Under which Article of the Indian Constitution did the President give his assent to the ordinance on electoral reforms when it was sent back to him by the Union Cabinet without making any changes (in the year 2002)?

The power to promulgate ordinances — including giving assent to them when they are presented for presidential action — is explicitly vested in the President under Article 123 of the Constitution. Article 123 authorises the President to issue ordinances during the recess of Parliament; such ordinances carry the force of law and are intended for urgent or unforeseen situations, subject to constitutional limitations described in standard texts [2]. Therefore, the President’s assent to the electoral reforms ordinance (after it was sent back by the Union Cabinet without changes in 2002) falls under Article 123.

Sources

1. [1] Indian Polity, M. Laxmikanth(7th ed.) > Chapter 18: President > ORDINANCE-MAKING POWER OF THE PRESIDENT > p. 197
2. [2] Laxmikanth, M. Indian Polity. 7th ed., McGraw Hill. > Chapter 18: President > ORDINANCE-MAKING POWER OF THE PRESIDENT > p. 198