How were the monguls able to bring almost all of india under one rule?

 Mongols "didn't bring all or part of India under one rule". Probably you are interpreting Mughals to mean Mongols, which is extremely tendentious reportage on your part, to hold any substance. The distance from India sharpened your ignorance, as India was never part of any Mongolian Empire. 
The only brush with Mongols (not any of its derivatives) was when Timur the lame (Timurlang or Tamerlane), overran the North Indian plains in 1299, pillaging, looting & killing, that people called him the great scourge. He never stopped anywhere but returned to the Central Asian liar. Whole Villages who were able to get advance information of his impending army raid used to run into the surrounding forests with just the clothes on, to save their lives from sure death. The brief interlude died as quickly as it started.

The correct answer is a