The correct answer is option 4 - the Narmada river flows through the trough of a rift valley inclined westward.

The Narmada river is known for its remarkably straight channel that flows westward in the Peninsular plateau. This straight channel is a result of the geology of the region. Option 4 explains that the river flows through the trough of a rift valley that is inclined westward.

A rift valley is a linear depression that forms when the Earth`s crust stretches and breaks apart. In this case, the Narmada river flows through the trough of such a rift valley. The westward inclination of the rift valley influences the flow of the river, causing it to follow a straight course in that direction.

Options 1, 2, and 3 do not accurately explain the reason for the remarkably straight channel of the Narmada river. Option 1 suggests that the slope gradient controls the river channel pattern, but it does not provide an explanation for the straightness of the channel. Option 2 suggests that the amount of water carried by the river has created a straight channel, which is not supported by evidence. Option 3 states that the river forms the boundary between the Central Highlands and the Deccan Plateau, but