The correct answer is option 3: Alluvial soil. The Northern Plain in India is a vast fertile region that stretches across several states including Punjab, Haryana, Uttar Pradesh, Bihar, and West Bengal. It is primarily formed by the deposition of sediments carried by the rivers originating from the Himalayas. This process of deposition over thousands of years has resulted in the formation of alluvial soil in the Northern Plain.

Option 1, laterite soil, is not the most common soil in the Northern Plain. Laterite soil is predominantly found in regions with high temperatures and heavy rainfall, such as the western coastal strip of India.

Option 2, regur soil, also known as black soil or black cotton soil, is found in the Deccan Plateau region of India and not in the Northern Plain.

Option 4, red soil, is commonly found in regions with a tropical climate and is not the most common soil type in the Northern Plain.

Therefore, the most common soil in the Northern Plain is option 3, alluvial soil, which is formed by the deposition of sediments from rivers.