Which is the best option after B.Tech for Civil Engineers - Job or Business?

Welcome to CE #-Link-Snipped-#‍ . The information you've provided is very limited can can't be used to give a specific answer. 

Whether you opt for job or business is totally up to you. If it's a family business and you wish to join it - I think that's always going to be a good option. Job opportunities are a function of the general trend in the industry and geo-location. 

A lot depends upon the kind of job you wish to do and the ones that are available for you to join. 

If you are looking to start a business of your own; have you made a plan on what you want to do, how you want to do it and how are you going to sustain it for the first year of operations? 

It'd help if you write in detail - what your current situation is and what is your exact dilemma. It'll help us offer better help to you.