Choose the sentence in which a relative clause comments on the whole preceding event.
The power failed that delayed the rehearsal.
The power failed, that delayed the rehearsal.
The power failed, which delayed the rehearsal.✓الإجابة الصحيحة
The power failed which delayed the rehearsal.
الشرح
Which refers to the entire fact that the power failed, and the comma marks this comment as non-defining. That is the closest distractor, but it cannot introduce a clause that comments parenthetically on a preceding statement.