The use of ellipses in the sentence is what makes it difficult …
Consider the following sentence :
Joe is afraid of ghost more than his wife ..
Can you make out about what this comparison is ??
Is Joe more afraid of ghost than he is afraid of his wife ??…or
Is joe more afraid of ghost that his wife is ??
In such cases, a repetition of “preposition” or “main verb” can clear the picture ..
For example :
Joe is afraid of ghost more than IS his wife …(notice the addition of “IS”)
Or Joe is afraid of ghost more than of his wife …(notice the addition of “OF”)…
Both are acceptable …so we need to understand the meaning and chose the option …
Now coming to the current question :
The Parthenon is larger than usual for a Doric temple, with eight columns on the short sides and seventeen on the long ones rather than the typical six and thirteen.
Currently, the sentence means that the Parthenon is larger than “USUAL” …but usual of what is not clear from the sentence …
The sentence is actually trying to convey that the Parthenon is larger than a usual Parthenon for a Doric temple..
This meaning can be clearly conveyed if we just repeat the “is”
The Parthenon is larger than “IS” usual for a doric temple ….which is OPTION B
Option D : one usually is :
As explained above, “one” is ambiguous and hence wrong…
Kudos if you like the concept explanation !!!
_________________