A ‘doab’ in Punjab refers to:

The correct answer is: a) A region between two rivers.

A doab is a region of land between two rivers. The word doab comes from the Persian word دو آب (do āb), meaning “two waters”. In the Indian subcontinent, doabs are often fertile agricultural areas. The most famous doab in India is the Doab between the Ganges and Yamuna rivers, which is home to the states of Uttar Pradesh and Haryana.

The other options are incorrect. A hill station is a town or resort located in the hills, often for the purpose of recreation or as a summer retreat. A desert oasis is a small area of vegetation in an otherwise arid desert. A coastal town is a town or city located on the coast.