The answer is (A.) The sail is pushing the wind back and slowing down the air. The wind is pushing the sail and the sail pushes the wind back and slows the air down.
As the boat is sailing, the wind will be faster than the boat. This fast wind is obstructed by the sail. That is, the fast wind gets slowed down by the sail because the sail pushes the wind backwards which slows down the wind. This will make the wind push the boat forward thereby accelerating the boat.
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