When you are replying to an email message in Outlook.com, the default option is set to Reply and not Reply All. But if you are frequently replying to more than one recipients then you can set the Reply All as the default option using Outlook.com Reply Settings.
Step 1: Click the Settings icon available at the right hand side and select More mail settings from the list.
Step 2: In Outlook.com settings screen, scroll down to Reading email section and click the Reply Settings option.
Step 3: In Make ‘Reply-all’ my default response section, set the option to “Yes” for Do you want to make “Reply-all” your default for responding to messages. Then click the Save button to confirm and save the changes.
Now the default Reply option in Outlook.com will be set to Reply All and you pick the Reply option from the drop down list.