In Gmail, the default option provided is reply and not reply to all. But using Google lab feature you can make reply to all as the default one.
Steps – Make reply to all as default in Gmail
Logon to your Gmail account and click the Settings link available on the right hand side.
Navigate to the Lab section.
Under Available labs, scroll down to Default ‘Reply to all’ section.
Select Enable radio option and click on the Save Changes button available at the top of the Lab screen. This would make reply to all as the default option in Gmail.