The correct answer is option 4: east to west and south to north.

To understand this, let`s imagine the Ganga basin as a large area on a map. The Ganga basin spans across multiple states in India, including Uttar Pradesh, Bihar, West Bengal, and parts of Madhya Pradesh, Uttarakhand, and Himachal Pradesh.

When we say "east to west," we mean that as we move from the eastern side of the Ganga basin towards the western side, the rainfall distribution pattern decreases. This means that the amount of rainfall decreases as we move from east to west. This could be due to various factors such as topography, prevailing wind patterns, and proximity to moisture sources.

Similarly, when we say "south to north," we mean that as we move from the southern part of the Ganga basin towards the northern part, the rainfall distribution pattern decreases. Again, this implies that the amount of rainfall decreases as we move from south to north.

Therefore, the correct answer is option 4: east to west and south to north, as it accurately describes the decreasing rainfall distribution pattern over the Ganga basin.