The correct answer is option 3: 1, 2, and 3 only.

First, let`s understand each option individually:

1. The valley of a river is widest in its lower course: This is correct because as a river flows downstream, it erodes the surrounding landscape more and widens its valley. The wider valley provides more space for sediments to accumulate and contribute to the choking.

2. The velocity of a river in its lower course is low: This is correct because as the river reaches its lower course, the gradient decreases, resulting in slower flow. When the velocity is low, the river lacks the necessary force to carry away the sediments, leading to their accumulation and choking of the river.

3. The delta sometimes develops in a river`s lower course: This is correct because a delta forms at the mouth of a river, where it meets a body of water like a lake or an ocean. The delta acts as a natural trap for sediments carried by the river, causing them to settle and choke the lower course.

4. Much of the river water is drawn for irrigation in the lower course: This option is incorrect and does not contribute to the choking of the lower course with sediments.

In summary, the reasons why the lower course