The answer is A. Because older seafloor is being subducted and destroyed.
Let's look into it one by one:
A. Because older seafloor is being subducted and destroyed. -This is correct.
-Besides constructive plate boundary that creat new plate as magma cools down,there are destructive plate boundaries that cause the denser oceanic plate to subduct under the less dense continential plate,destroying the pate and keeping the size of the floor.
B. Because the earth is simultaneously getting smaller. - This is false.
-There is no scientific proof that the earth is getting smaller.
C. Because the continental plates are getting larger, too. - This is false.
Although there are indeed constructive plate boundaries on land,but they mostly locate at the oceanic plate thus the construction of plate is not significant.
D. Because hot spots form magma is released.-This is false.
-Hot spot only create volcanoes or submarine volcanoes,it is insignificant on enlarging the continential plate.
Hope it helps!