Can you please tell me any thing will happen if i cannot choose the medium of sendind acknowledgement to cpc?

No, 

Problem won't arise if you send Acknowledgement through post. Problem would arise if the acknowledgement sent through post doesn't reach CPC Bangalore. Notice will be send by CPC through E-Mail or SMS or letter would be sent to the location mentioned in the Acknowledgement. 

The Acknowledgement is the proof that you know the details filed and you have confirmed the details mentioned in the Acknowledgement and the process of Assessment can be started by CPC. 

E-Verfication is also using the same principle ie confirmation from the Assessee.

Please note that after generation of Acknowledgement, you have to submit it to CPC before 120 Days of generation.

Hope this information helps you.