(B) India, vast in size and bounded off from the outer world by 

(A) the seemingly impenetrable mountain ranges constituting her northern boundary

 (D) naturally developed from early times the notion that she is a world by herself

 (C) unsullied by extraneous contaminations

The reason the correct order is (3) (B), (A), (D), (C) is that it forms a logical and coherent paragraph. It starts with an introduction of India's geographical isolation, moves on to describe the northern boundary, and then discusses the notion that India is a world by itself naturally developed over time, concluding with the idea that it is unsullied by extraneous contaminations. This order ensures that the sentences flow smoothly and make sense when read together.