The correct option is D UUUUUU
The first term is having only one letter.
Second term is having two letters, third term is having four and fourth term is having four letters.
So, sixth term should contain six letters.
Z is the first alphabet starting from the last, Y is the second, X is third, W is fourth.
Therefore, the sixth alphabet from the end is U.
So, writting this letter for 6 times gives us out sixth term of the series.
Therefore, the sixth term is UUUUUU.