Final answer:
The abomasum is not a forestomach but the true stomach where digestion occurs with gastric enzymes, making option C the correct answer.
Step-by-step explanation:
The ruminant digestive system is unique for its multi-chambered stomach which includes the rumen, reticulum, omasum, and abomasum. These chambers are critical for the digestion and fermentation of plant material due to the presence of microbes that break down cellulose. Among these, the abomasum is not considered a forestomach because it functions as the true stomach, akin to the monogastric stomach chamber, where actual digestion with gastric enzymes takes place. Therefore, the correct choice that is NOT considered one of the fermenting forestomaches is C. Abomasum.