To ensure that only a part of the Visualforce page is refreshed, developers can use the reRender attribute of the <apex:commandButton> tag. This attribute specifies the IDs of the components that should be re- rendered as a result of the action method being completed. In this case, setting the reRender attribute to the ID of the <apex:pageBlockSection> that contains the opportunity list will cause only that section to be updated on the page, resulting in a partial page refresh.
The correct answer is B because it uses the reRender attribute correctly to update only the specific part of the page (opportunityList) without a full page refresh.
References:
Salesforce Developer Documentation on Visualforce AJAX Components: Visualforce Developer Guide - CommandButton