The most beneficial historical source for a historian researching a loyalist’s experience of the Boston Massacre would be a personal letter written by a British soldier stationed in Boston to their family in Great Britain. This source would likely provide direct insights into the perspectives and experiences of someone aligned with the loyalist cause during that specific event.
The other options either come from perspectives that do not represent loyalist views (like the diary entry by Attucks or the pamphlet by the Sons of Liberty), or are secondary analyses that may not reflect firsthand experiences (like the scholarly article).