The correct answer is: D. 600 m to 1500 m.
The foothills of the Himalayas are a transitional zone between the mountains and the plains. They are characterized by a series of parallel ridges and valleys, with an average elevation of 600 to 1500 meters. The foothills are home to a variety of plant and animal life, and are an important region for agriculture and human habitation.
Option A is incorrect because the foothills of the Himalayas are much higher than 300 to 400 meters.
Option B is incorrect because the foothills of the Himalayas are much higher than 400 to 500 meters.
Option C is incorrect because the foothills of the Himalayas are much higher than 600 to 900 meters.