Whether a new user of my old number can see my old WhatsApp messages? I was using WhatsApp with my mobile number since last 1 year. Suddenly the number got deactivated due to no call/messages for long days. However, I got to know that the same number has been allotted to another person-B. 

Now I am wondering whether the person-B can be seeing my old WhatsApp messages those I had 1 year before attached to the same number?
They cannot see your messages because they would not be able to sign into your account. Without your password and/or physical access to your phone where your messages are saved there is no way they will be able to access your messages.

An entirely new account can be created for the new owner of that number, but it is separate from your old account and does not have access to your old messages.