If the tree was 3 feet tall when Mr. Hooper planted it 6 years ago, and it grows every year, we need to determine the current height of the tree. Since it grows every year, the height would have increased by 6 feet (6 years multiplied by 1 foot growth per year).
Therefore, the current height of the tree in terms of "f" would be 3f + 6 feet. So the correct answer is A. 3f + 6 feet.