The correct option is A Endoplasmic reticulum
Endoskeleton refers to the internal membrane system of a cell. The internal membrane system supports the cells like a skeletal system supports the body of an organism. The endomembrane system comprises of endoplasmic reticulum, Golgi apparatus, lysosome and plasma membrane. Endoplasmic reticulum (ER) is a system of membrane-lined channels found in all eukaryotic cells except mature erythrocytes. It constitutes more than 30 to 60 percent of total cell membranes and is the major component of the endomembrane system. Thus, the correct answer is option B.