In what action is the depressor anguli oris muscle primarily involved?

The depressor anguli oris muscle plays a key role in facial expressions, particularly in the action of frowning. This muscle is located at the corners of the mouth and is responsible for pulling the corners of the mouth downwards. When this muscle contracts, it creates a downward movement that typically indicates sadness, discontent, or displeasure, which is commonly associated with frowning.

In contrast, the other actions listed rely on different muscles: smiling involves the zygomaticus major and other muscles that elevate the corners of the mouth, eating engages various muscles for chewing and positioning food, and speaking involves a coordinated effort of several facial muscles for the movement of the lips and mouth. Thus, the depressor anguli oris is specifically linked to the expression of a frown, making it the correct choice in this context.