An antecedent drainage refers to a river system that existed before the formation of a particular geographical feature, such as a mountain range or a plateau. It refers to a river that has maintained its original course and flow pattern despite the upliftment of land or the formation of new geological structures.

Let`s dissect each option to understand the correct definition of an antecedent drainage:

Option 1: "Which follows the initial slope of the Himalaya"

This option implies that the river follows the initial slope of the Himalaya, which may not necessarily indicate that it existed before the mountain range formed. It does not accurately define an antecedent drainage.

Option 2: "Which existed before the Himalayan range came into existence"

This option correctly defines an antecedent drainage as a river system that was present prior to the formation of the Himalayan range. The Brahmaputra and Indus rivers being antecedent rivers means that they existed before the formation of the Himalayas.

Option 3: "Which followed the dip or rock beds of the Himalaya"

This option implies that the river followed the dip or rock beds of the Himalaya, but it does not accurately define an antecedent drainage. The term "antecedent" refers to the river being