How is the pitch of a sound in a guitar changed if:

(a) a thin wire is used

(b) a wire under less tension is used?

(a) Pitch of sound will be higher if a thin wire is used in a guitar.
Thin wires in stringed musical instruments produce sound with higher frequency and hence higher pitch.

(b) Pitch of sound will be low if a wire under less tension is used in guitar.
In stringed musical instruments, wires under less tension produce sound with lower frequency and hence lower pitch.