Answer:
FLAW
Step-by-step explanation:
A FLAW signifies or denotes an imperfection, fault, or mistake. It means something deviates from what is adjudged or deemed to be okay.
The most appropriate word that fits in well in the sentence above is "FLAW". Already we have a clue given already in the form of 3 letters of which 1 letter is required to complete the word which fits in best.
The ballet instructor when critiquing a performance would look for flaws or fault in the performance which would form his basis for rating how perfect a performance is.