Tissues are classified on the basis of their structure and function.
Connective tissue (B), as exemplified by Cartilage (D), has a characteristic extracellular matrix.
Muscle tissue (C) is typically innervated with blood vessels.
Nervous tissue (E) are made of neurons, which are not generally found on the inside surface of stomach.
Therefore, options B through E are incorrect.
Epithelial (A) cells of various morphologies form the lining of most organs and they are devoid of extracellular matrix and these cells need not be directly innervated by blood supply. Therefore, such cells could be the ones studied in the context, under microscope.
Therefore, option (A) is correct.