Labio-dental sounds are a type of consonant sound in phonetics. The name itself gives a clue: 'labio' refers to the lip, and 'dental' refers to the teeth.
These sounds are produced by bringing the lower lip into light contact with the upper front teeth. Air is then expelled through the small gap created between them, causing friction.
Based on the definition and articulation process, the articulators used for labio-dental sounds are the lower lip and upper teeth. This corresponds to Option 3.