Explanation:

Small hydro is often developed using existing dams or through development of new dams whose primary purpose is river and lake water-level control, or irrigation.  So, small hydro projects require flowing streams, better if streams are perennial and flow at a fast speed. Himalayan region provides a perfect potential ground. So, (a) is correct.
In Coastal region, stream velocity becomes very low, so (d) can’t be the answer.
There are not many streams in Central India and Western India when we compare them to Himalayan states. So, (c) and (b) aren’t the answer. Also, states where irrigation canals are dominant small hydro can be generated