Probably many of you noticed the pattern here.
At Dimension-1, we had two to the one (2^1), or two (2) copies.
Each of them were one-half size, or one over two to the one (1/(2^1) = 1/2) of the previous level.
This is when we were bisecting the line.
At Dimension-2, we had four (4) or two to the two (2^2) copies at one-fourth size, or one over two squared (1/(2^2)=1/4).
At Dimension-3, we had two to the three (2^3) copies of one over two to the three (1/(2^3)=1/8) size, and so on.
So for Dimension-D, we would have, well, two to the D (2^D) copies of the object at the previous level, and
each one of them would be of size one over two to the D (1/(2^D)), so
for twenty (20), the answer would be "D" here.