The correct answer is (a) Uttarkashi. Uttarkashi district has the maximum forest area in Uttarakhand, with 75.9% of its total area under forest cover. The district is located in the Garhwal Himalayas and is home to several important rivers, including the Ganges, Yamuna, and Alaknanda. The forests of Uttarkashi are home to a variety of wildlife, including snow leopards, tigers, and elephants.
The other options are incorrect for the following reasons:
(b) Dehradun is the capital of Uttarakhand and has a forest cover of 34.7%.
(c) Haridwar is a holy city on the banks of the Ganges and has a forest cover of 20.2%.
(d) Nainital is a popular tourist destination and has a forest cover of 41.8%.