Which one of the five options (A to E) fits in the blank space? Why?
The answer is:
E - the larger 'U A U'.
Each rectangle displays the vowels from the name of a major body in our solar system. Specifically, starting top left and going clockwise, we see the vowels from the eight planets in order, going around the central rectangle containing the vowel from the Sun!
Moreover, the relative size of the letters in each box roughly represents the relative sizes of these bodies (not to scale).
In other words, we have (going clockwise):
MERCURY, VENUS, EARTH, MARS, JUPITER, SATURN, ???, NEPTUNE.
URANUS! i.e. 'U A U'. Considering the two sizes available, we need to take the one more similar in size to Neptune (E U E) rather than the one close in size to Earth (EA) - i.e. option E.
(Also note that when vowels appear consecutively in a name - e.g. in 'EArth' - the letters appear next to each other without a space in this rebus. This is another reason to choose the more 'gappy' 'U A U' over the consecutive-appearing 'UAU' option in A...)