The most beneficial source for a historian researching a loyalist’s experience of the Boston Massacre would be a scholarly article analyzing the goals of loyalists during the American Revolution.
This source would provide contextual insights into the motivations, thoughts, and reactions of loyalists during that period, including their perspectives on events like the Boston Massacre. The other options, while valuable, either represent perspectives that are not loyalist (such as the diary of Attucks and the pamphlet by the Sons of Liberty) or provide a limited viewpoint (the personal letter from a British soldier primarily would reflect a soldier’s perspective rather than that of a loyalist civilian).