Which is a better option: IIT Guwahati CSE, IIT BHU CSE, IIT Bombay mech or IIT Kharagpur electrical and electronics communication?

I assume that you haven’t decided on a specific branch and hence you have multiple options open from different institutes and branches. Before I give you my suggestion it’s important to know that most non-CS students at IIT Bombay and IIT Kharagpur try to get a job in CS only. For this, they learn programming and sit for non-core placements. 

For the same reason, I am recommending you join either BHU or Guwahati (CSE).

• BHU is known for its elaborate and old alumni base leading to better placements whereas Guwahati speaks highly of its infrastructure.
• BHU has much better connectivity owing to its city locations whereas Guwahati is situated outside the city.
• Guwahati has a larger faculty pool than BHU

In the end unless you are a North-Eastern, BHU is the best option for you. Either way you are good to go and thus can make a decision that suits your location.