Final answer:
The cecum, a pouch forming the beginning of the large intestine, is the anatomic structure similar in rodents and horses, aiding in the fermentation of plant materials(OPTION A).
Step-by-step explanation:
The anatomic structure that is similar in rodents and horses is the A)cecum.
The cecum is a pouch forming the beginning of the large intestine and plays a crucial role in the fermentation of plant materials. Both rodents and horses have well-developed ceca, which allow them to break down cellulose in plant matter through bacterial fermentation. Other common organs like the spleen, esophagus, and rumen (specific to ruminants, not horses) do not share the same level of similarity in function or development between horses and rodents as the cecum does.