Which course should I opt after completing B.Tech in mech?

Like harshit said, you can go for automobile engineering / thermal or robotics engineering. They all have good scope in our country. Colleges where you can apply for these courses are: IITs Anna university, Chennai Hindustan institute of engineering technology, Chennai PEC university of technology,

You can opt for M.Tech in automobile engineering, robotics engineering or in thermal systems. These courses have a good scope. Automobile engineering has more scope in mechanical. Also you can choose mechatronics which has a less scope in India but if done from a good Institute then a lot of options