What is an example of a bilabial sound?

A bilabial sound is one that is produced using both lips. In the context of the given choices, the sound represented by the letter "M" is a bilabial nasal. To articulate this sound, the lips come together to block airflow and then the velum lowers to allow air to escape through the nose, producing the "M" sound.

In contrast, the other sounds listed are not bilabial. The sound associated with "F" is a labiodental sound, which involves the lower lip and the upper teeth. The sounds for both "T" and "D" are alveolar, meaning they are produced with the tongue against the alveolar ridge, which is the bony ridge behind the upper front teeth. Thus, "M" stands out as the correct example of a bilabial sound among the options provided.