The correct answer is (a).
According to the census of 2011, the female literacy rate in Udhamsingh Nagar was 84.8%, in Pithoragarh was 82.9%, and in Rudraprayag was 79.2%. Therefore, the correct descending order of the districts of Uttarakhand on the basis of female literacy rate is Udhamsingh Nagar, Pithoragarh, Rudraprayag.
Option (b) is incorrect because the female literacy rate in Pithoragarh is higher than the female literacy rate in Rudraprayag.
Option (c) is incorrect because the female literacy rate in Udhamsingh Nagar is higher than the female literacy rate in Rudraprayag.
Option (d) is incorrect because the female literacy rate in Rudraprayag is lower than the female literacy rate in Pithoragarh.