The procedure that uses liquid radiopaque contrast medium inserted into the rectum and colon is:

The right answer is barium enema.

The barium enema is a radiological examination that allows studying the rectum and colon. Its purpose is to detect inflammation, infection, polyps or tumors. It uses x-rays and a baryte-based contrast product.
Its principle is to opacify and visualize the large intestine. The barite is introduced into the colon by a small cannula placed in the anus. It progresses throughout the colon and lines its walls that will be visible on the pictures thanks to the opaque radio properties of this product.