Answer:
The main source
Step-by-step explanation:
The person writing this genre of literature usually is a known or famous personality whose life many people may have great interest or curiosity to know about. A biography offers a general content about their history, but the memoir is about a specific event which might have details that only the person themselves is able to remember and inform, or if there are other writers, the person knows better, and is the main source just because it's their experience (and their responsability of being true to their audience.)