What is an excision of a small piece of tissue for microscopic examination called?

The correct answer is biopsy. A biopsy involves the removal of a small piece of tissue from a living organism for diagnostic purposes, particularly to identify diseases such as cancer. This procedure allows pathologists to examine the tissue microscopically, providing important information about the presence of any abnormalities or disease processes.

In the context of the other options, an incision refers to a surgical cut made in the skin or tissue but does not imply the removal of tissue for examination. An autopsy is the examination of a body after death, typically to determine the cause of death, and is not related to the assessment of living tissue. Necropsy is often used interchangeably with autopsy, primarily in veterinary medicine, to describe the examination of deceased animals. Neither autopsy nor necropsy pertains to the examination of living tissue like a biopsy does. Therefore, biopsy is the most appropriate and specific term for the excision of tissue for microscopic examination.