By default the O365 reply settings are set to "Reply All".
1. You can click the arrow to the right to show the available options.
2. Select "Reply"
If you would like to set "Reply" as default please do the following:
1. Click the gear icon at the top of the page then click "Mail" located under "Your app settings" .
2. Located under "Mail" click "Reply Settings" and select the "Reply" option and Save.
3. Your default should now look like this.