Final answer:
A speculum is the instrument used to expand and separate the walls of a cavity to make examination possible.
Step-by-step explanation:
The instrument used to expand and separate the walls of a cavity to make examination possible is a speculum. A speculum is a medical device that is inserted into a body cavity or opening to facilitate examination or perform procedures. It is commonly used in gynecology to examine the cervix and vagina. By expanding and separating the walls of the cavity, a speculum allows for better visualization and access to the area being examined.