Hi!
The question states: the height of a tree twenty feet tall increases by two feet each year.
Since it increases at a constant rate, 2 feet per year, it's linear.
Linear equations, when graphed, are a straight line, because the same amount is added at a constant rate. The constant rate in this case is 2 feet per year.
Quadratic is something completely different. It's a curve, going up, and then going down. A problem that would apply to that would be, for example, throwing a ball upwards.
Exponential, is when something is multiplied at a steady rate. If this question was exponential, it would have to be, the height of a tree twenty feet tall increases by double each year.
So your answer is linear.
Hope this helped!