The correct answer is: a) The Himalayas.
The Himalayas are a mountain range that runs along the northern border of India. They are the highest mountain range in the world, and they have a significant impact on the climate and weather patterns in India. The Himalayas block the cold air from Central Asia, which helps to keep India warm. They also cause the monsoon winds to blow from the south, which brings heavy rains to India during the summer months.
The drainage pattern in Arunachal Pradesh is largely influenced by the Himalayas. The Himalayas cause the rivers in Arunachal Pradesh to flow from north to south. The rivers in Arunachal Pradesh are also very fast-flowing, which is due to the steep slopes of the Himalayas.
The other options are incorrect because they do not have a significant impact on the drainage pattern in Arunachal Pradesh. The monsoon winds are important for the climate and weather patterns in India, but they do not have a direct impact on the drainage pattern. The Western Ghats and the Thar Desert are located in the southern and northwestern parts of India, respectively. They are not located near Arunachal Pradesh, so they do not have a significant impact on the drainage pattern in Arunachal Pradesh.