Where in the body would you find smooth muscle?

Smooth muscle is found in the walls of hollow organs throughout the body, such as the intestines, bladder, blood vessels, and stomach. This type of muscle is responsible for involuntary movements, allowing these organs to function automatically without conscious control. For example, smooth muscle contracts to move food through the digestive system or to regulate blood flow by constricting or dilating blood vessels.

In contrast, the other options indicate locations where other types of muscle are found. The skin primarily consists of connective tissues, epithelial cells, and does not contain smooth muscle. The brain is made up of nervous tissue and neurons, and while it does control many involuntary functions, it does not contain smooth muscle itself. The heart contains cardiac muscle, which is a distinct type of muscle specialized for the heart's rhythmic contractions.

Thus, the presence of smooth muscle in the walls of hollow organs is what makes this choice the correct answer.