Answer: Ok, the acceleration decreases means if you have an object with acceleration equal 100 meters for second square, and it starts to decrease slowly to 90, 80... etc, there you still have positive acceleration, so your object's velocity keeps increasing, but more slower than before.
remmember, acceleration means the change in velocity, if you have positive acceleration, your velocity will increase.
the correct answer is D, the air resistance will fight against the gravity, but the object will keep accelerating (with less intensity) ence will go faster and faster.