The Aravalli range is the oldest ___________ geological feature anywhere in the world. These were once tall mountains, possibly as high as the Himalayas. Over hundreds of millions of years these tall mountains have been___________ to low hills and ridges. Yet, these diminutive hills have been ___________ to many important events in Indian history.

The paragraph presents to us a scientific context, and the terms should echo the same. Both oldest 'existing', or better, oldest 'surviving' are good choices. Both 'eroded' and 'broken down' are good choices in the second blank. And in the third blank, only the words 'witness' and 'observers' are sensible choices. The answer should therefore be option C, as it includes all the good choices. Option A uses the word 'sentinel' for the last blank and is incorrect, option B similarly uses 'testament', even 'testaments' would've been somewhat fine. Option D uses 'continuing' in the first blank, which is not as good as 'surviving'.