Option 4, Narmada—Tapi—Mahi—Sabarmati, is the correct order of west flowing rivers in terms of decreasing size of their river basins.

To understand why this order is correct, let`s look at each option individually:

Option 1: Narmada—Tapi—Sabarmati—Mahi

This option is incorrect because it starts with the Narmada river, which has the largest river basin, but then incorrectly places the Sabarmati river before the Mahi river. The correct order should have Mahi before Sabarmati.

Option 2: Tapi—Narmada—Mahi—Sabarmati

This option is also incorrect because it starts with the Tapi river, which is not the largest in terms of river basin size. The correct order should begin with the Narmada river.

Option 3: Sabarmati—Narmada—Tapi—Mahi

This option is incorrect because it starts with the Sabarmati river, which has a smaller river basin compared to the Narmada river. The correct order should have the Narmada river first.

Option 4: Narmada—Tapi—Mahi—Sabarmati

This option is correct because