The correct answer is: a) High Himalayan ranges.
The northernmost physiographic zone of Arunachal Pradesh is characterized by high Himalayan ranges. The Himalayas are the highest mountain range in the world, and they form a natural barrier between India and China. The Himalayas are home to many glaciers and snow-capped peaks, and they are a popular destination for mountaineering and trekking.
The other options are incorrect because they do not accurately describe the northernmost physiographic zone of Arunachal Pradesh. Option b, rolling hills, is incorrect because the northernmost physiographic zone is characterized by high mountains, not rolling hills. Option c, river valleys, is incorrect because the northernmost physiographic zone is not home to any major river valleys. Option d, coastal plains, is incorrect because the northernmost physiographic zone is located in the Himalayas, which are far from any coastal plains.