1) The musicians is incorrect, because most adjective clauses start with variations of "who" and "where" and "when" and "why" etc...
2) Gave a stellar performance is incorrect for the same reason
3) Who had been practicing is incorrect because it doesn't elaborate on "practicing"
4) Who had been practicing for months is correct because it tells how long they were practicing, and elaborates on the adjective. (practicing)
The musicians, who had been practicing for months, gave a stellar performance. The phrase which is the adjective clause in the sentence is - 'gave a stellar performance' and 'who had been practicing for months'. It is because the word stellar has been used as the adjective as it modifies the performance and practicing for months is used to tell how long they were practicing.
Adjectives are the words which modifies noun.
Stellar refers to good performance. It also means the stars.
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