Final answer:
The question is about American Sign Language (ASL) and involves signing the letter 'Z'. It also touches upon English phonics and hand anatomy related to extensor expansions.
Step-by-step explanation:
The question you're asking relates to American Sign Language (ASL), which is a visual language used by the Deaf and hard-of-hearing communities. In ASL, the letter 'Z' is signed by drawing it in the air with your index finger.
To underline the letter that represents a particular sound in a word, such as underlining the letter that spells [k] in 'quiet', which is 'qu', or the letter that spells [g] in 'government', which is 'g', is an exercise in phonics, often encountered in English language learning.
The description regarding extensions over the phalanges seems to be about hand anatomy, specifically the extensor expansion, which plays a role in finger movements.