Final answer:
The sensation detected when the rectum fills with feces is mediated by autonomic fibers, specifically those in the parasympathetic division of the autonomic nervous system involved in the defecation reflex (option B).
Step-by-step explanation:
The fibers that detect sensation when the rectum fills with feces are the autonomic fibers. In particular, it is the parasympathetic division of the autonomic nervous system that mediates the defecation reflex, a process initiated by the stretching of the rectal wall as it fills with feces.
This reflex involves the contraction of the sigmoid colon and rectum, relaxation of the internal rectal sphincter, and initial contraction of the external rectal sphincter. The parasympathetic nervous system, through its activation via the spinal cord, is crucial for this involuntary aspect of the defecation process.
Hence, the answer is option B.