Respuesta :
The assumption that the narrator make in the excerpt is: B. that the messages would increase his mother’s suffering
A statement would be considered as assumption if it based on personal's train of thought without accompanied by facts.
From this line:
I knew I could never let Mom hear the messages, because protecting her is one of my most important . . .
We can infer that the speaker believe that hearing the message would somehow cause some sort of damage to the mother, which is why the narrator choose to not give the message.