adenocarcarcinoma
Microscopically, look at a region where you can see both malignant and benign elements in the same field. The difference in gland and cellular morphology will be much more evident. The malignant glands show "gland within gland" pattern as well as disorganized arborization and branching. The usual cellular features of malignancy are here in abundance: nuclear-cytoplasmic ratio, hyperchromasia, angulated nuclear margins etc.