The correct answer is:
Grace’s opponent is correct, and it counts as a fault.
From ITF Rule 19:
The service is a fault if the server misses the ball when trying to hit it. If Grace sneezed during her toss and didn’t attempt to hit the serve, then she would have been correct, and it wouldn’t have counted as a fault.
Furthermore, since the sneeze was within her control, she cannot call a hindrance.
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