The correct answer is: Both A and R are true, but R is not the correct explanation of A.
The assertion (A) is true. The river valleys in the Western Himalayas between 1500 and 3300 m have moist temperate forests. The reason (R) is also true. Pine, cedar, fir, and spruce are some of the trees that grow well in these forests. However, R is not the correct explanation of A. The reason for the presence of moist temperate forests in these river valleys is the high rainfall and moderate temperature. The rainfall in these valleys is about 1500-2000 mm per year, and the temperature ranges from 10-20 degrees Celsius. These conditions are ideal for the growth of moist temperate forests.
The other options are incorrect. Option A is incorrect because R is not the correct explanation of A. Option B is incorrect because R is not the correct explanation of A. Option C is incorrect because R is true. Option D is incorrect because A is true.