Final answer:
The term for a letter like P or D where the mouth is partially blocked to make the sound is called a 'Stop', which is option B.
Step-by-step explanation:
A letter like P or D, where some part of the mouth is blocked off to make the sound, is called a mute. These types of letters are associated with a cessation of airflow in the vocal tract. The correct term for this simple word of cessation is Stop, which is option B. A mute, as defined in the context of phonetics, refers to a consonant sound where the flow of air is fully stopped at some point in the oral cavity.
For example, the letter P in words like 'perfect' and 'pop' or the double P spelling as in 'stopped' and 'tripped' are all instances of the sound [p] where the airflow is stopped by the lips. Similarly, the letter D, as found in 'started' or 'wonderful,' is a mute consonant where the sound is created by stopping the airflow with the tongue against the upper palate.