Question

find the relative clause. if it is nonrestrictive, set it off with commas. if it is restrictive, submit the text without making any changes.
the jazz quartet hired pablo who has perfect pitch to contribute backup vocals to its recording of \stardust.\

Explanation:

The relative clause here is "who has perfect pitch". It provides non-essential, extra information about Pablo (we already know which person, Pablo, is being referred to, so the clause just adds a detail). Nonrestrictive relative clauses need to be set off with commas.

Answer:

The jazz quartet hired Pablo, who has perfect pitch, to contribute backup vocals to its recording of "Stardust."