souvonik2k
Japan has a land area
similar to California but fully 128 million in population, where 23% are over the age of 65, according to the 2010 census.
A) about the same as California but fully 128 million in population, where 23%
B) of a similar size as California is, but in Japan there is a population of fully 128 million, and
C) that is about the same size as California's land area, but in Japan with a population of fully 128 million people, of them
D) similar to the size of California, but fully 128 million in population, and
E) similar to that of California but a population of fully 128 million people, of whom
Source-Gmatfree
I am sorry but this is clearly not a GMAT style question. Option A clearly does not match the underlined portion.

.... So essentially we have 6 options

Now coming to the Analysis Part : According to 2010 census,
1. As per the intended meaning Japan's land area is to be compared with California's land area.
2. Then the author presents a contrast word 'But'.
3. Then the sentence says that Japan has 128 million in population
& finally 4. 23% of that population is over the age of 65
Option A: Incorrect : Japan's land area is compared with California.
but fully 128 million in population quite an awkward construction.
Where generally should be used modify a place.
Option B: Incorrect :
California is is not correct .... most relevant in this context is
California has. Now in the sentence we donot know
23% of what?
Option C: Incorrect : Lengthy construction.
,of them is non-idomatiic.
Option D: Incorrect : Similar issue as A:
but fully 128 million in population quite an awkward construction. Similar issue as B :Now in the sentence we donot know
23% of what?
Option E: Correct : All the concerns have been addressed