Which of the following sounds is an example of a bilabial sound?

The bilabial sounds are produced by bringing both lips together. In this case, the sound represented by the letter "b" is a bilabial sound because it is articulated by closing both lips and then releasing them while vibrating the vocal cords. This is why "b" is the correct answer.

To provide context for the other sounds: the sound represented by the letter "f" is produced by bringing the upper teeth into contact with the lower lip, which categorizes it as a labiodental sound. The "th" sounds, such as those in "think" or "this," are produced with the tongue placed between the teeth, making them dental sounds. The "s" sound, often produced with the tongue close to the alveolar ridge, is classified as an alveolar sound. Understanding these distinctions is crucial for recognizing how different types of sounds are articulated in speech.