The correct answer is option 'A', the Himalayan mountain range, as it defines the northern frontiers of India.

The Himalayan mountain range is a massive mountain range that spans across several countries including India, Nepal, Bhutan, and Tibet. It is considered to be one of the most significant geographical features of India and plays a crucial role in defining its northern boundaries.

Here is a detailed explanation of why the Himalayan mountain range defines the northern frontiers of India:

1. Geographical Location: The Himalayas are situated in the northern part of the Indian subcontinent and act as a natural barrier between India and its neighboring countries. This mountain range stretches over 2,400 kilometers from the western end of India to the eastern tip, forming a natural boundary that separates India from countries like China and Nepal.

2. Highest Peaks: The Himalayas are home to some of the highest peaks in the world, including Mount Everest, Kanchenjunga, and Nanda Devi. These majestic peaks not only provide a stunning landscape but also serve as a physical boundary that demarcates the northern frontiers of India.

3. Strategic Importance: The Himalayas have significant strategic importance for India. They provide a natural defense against potential invasions from the north, acting as a shield for the country. The rugged terrain, high altitude, and extreme weather conditions make it difficult for any military forces to easily cross over into Indian territory.

4. River Origins: Several major rivers in India, such as the Ganga, Brahmaputra, and Indus, originate in the Himalayas. These rivers play a crucial role in India's agriculture, economy, and culture. The Himalayas act as a watershed, directing the flow of these rivers towards the Indian plains, thus further emphasizing their significance in defining India's northern frontiers.

5. Biodiversity: The Himalayas are known for their rich biodiversity, with a wide variety of flora and fauna found in the region. This unique ecosystem is home to several endangered species and serves as a vital ecological corridor. The conservation of this biodiversity is of utmost importance to India, and the Himalayas act as a natural boundary for protecting these fragile ecosystems.

In conclusion, the Himalayan mountain range defines the northern frontiers of India due to its geographical location, highest peaks, strategic importance, river origins, and rich biodiversity. It acts as a natural barrier, providing defense, and separating India from its neighboring countries.