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.
Option Elimination -
Bit of a history that is not necessary but good to understand to clarify the concepts.
The name of the Parthenon comes from the Greek word Parthenos, which means "virgin" or "innocent maiden." The Parthenon is a Doric temple built in the 5th century B.C. on the Acropolis in Athens. It (The Parthenon) was built as a temple to the goddess Athena Parthenos. It (The Parthenon) was also used as a treasury for the Delian League, a confederacy of ancient Greek states.
The Doric order was a style of architecture that originated in Greece in the late seventh century B.C.E. The Doric order is characterized by simple circular capitals at the top of columns. Think of it as some architectural guidelines.
So, the intended meaning is that "The Parthenon" is a Temple built during the time when Dorc order was in place. Say, for example, the Doric order said, kindly build temples in a 10x10 size. But say the Parthenon was 100x100. So, the author tries to bring up the unusualness of "The Parthenon." If "The Parthenon" had followed the typical Doric order, the temple size would have been smaller. But "The Parthenon" seems to be a significant departure from the Doric order (architectural guidelines of that time), and that's what the author wants to highlight.
A bit on the usage of "usual" and "is usual."
"The burger is bigger than is usual." (If you use some grammar tool, then "is" in this example will show underlined. So it's a silly example just for understanding. But this sentence, "The temple is larger than is usual for a Doric Temple," "is" will not be underlined (at least that's what I am seeing now

) There is a reason why it is not highlighted, and the grammar tools don't consider it as wrong while in burger example "is" may be wrong.
So, back to our "burger" example. This sentence is a more formal construction using the word "is" after "than." "than is usual" is a subordinate clause that compares. This formal construction implies that there is a specific standard, expectation, or average size for the burgers, and the burger being referred to exceeds that norm. Using this construction, the sentence emphasizes the comparison and suggests that there is a recognized or established standard for burger sizes.
"The burger is bigger than usual.". This sentence is less formal and used in everyday speech. It states that the burger is larger than what is typically expected or experienced without explicitly referencing a specific standard or a norm. "Wow, this burger is huge! It's definitely bigger than usual". This construction is more relaxed and suitable for informal conversation or casual writing.
In summary, both sentences are grammatically correct but convey slightly different nuances. The first is more formal and suggests a recognized standard, while the second sentence is more casual.
Sorry for a weird example on burgers

. I hope it helps to understand the difference between options A and B.
So for option B, we are comparing the "The Parthenon," a temple, with a temple built as per the standard, which is the Doric order; thus, we use "is usual" as a more formal construction.
(a) usual - grammatically ok but not ok for a more formal comparison here against the established Doric order of those times. Another way to look at it is "The Parthenon is larger than usual for a Doric temple." "usual" functions as a predicate adjective, giving more information about the subject, which is "The Parthenon" here. So the expansion looks like "The Parthenon is larger than usual for a Doric temple." For example, the company's revenue growth this quarter was higher than usual. So, the "usual" or customary revenue growth may be less, but this quarter is higher than usual.
If we keep options A and B side by side - The Parthenon is larger than usual for a Doric temple" and "The Parthenon is larger than is usual for a Doric temple." First, a level set; both are grammatically correct. But there is a subtle difference.
The difference lies in the emphasis of the comparison. The first sentence emphasizes the Parthenon's size in relation to the standard or norm for Doric temples. The second sentence places greater emphasis on the idea that the Parthenon deviates from what is typically expected or experienced in terms of size for a Doric temple.
Both sentences convey the same general meaning that the Parthenon is larger than what is usual or expected for a Doric temple. However, the second sentence, with its use of "is usual," may imply a stronger deviation from the norm or a greater departure from what is typically encountered in Doric temples.
(b) is usual - ok. Put simply, the intended comparison is that "The Parthenon is usually larger than a typical Doric temple (built in compliance with the Doric order)." So, if you picked any other temple when the Doric order was in effect compared to the typical temple designed as per the Doric code, The Parthenon (built during the Doric order age) is more extensive.
We are comparing the largeness of "The Parthenon" with the size of any other Doric temple built as per Doric order.
(c) has been usual - the present perfect tense is primarily used to indicate actions or states that started in the past and are connected to the present. It does not typically convey information about the continuity of the action or state into the future. So are we saying that largerness than usual will not be applicable in the future? No right? So, we need the present tense for the perpetual truths.
(d) one usually is - "one" refers to "The Parthenon." So are we saying, "The Parthenon is larger than The Parthenon for a Doric temple"? It doesn't make sense. Is "The Parthenon" some architectural feature? No? It is the temple or the architecture itself. Even in that non-sensical meaning, it has to "The Parthenon of X temple is larger than The Parthenon of the Doric temple." This is a total mess.
(e) it is usual - E.g., "During rush hour, it is usual for traffic to be heavy."
In this example, "it is usual" expresses a standard or typical occurrence. The phrase "it is usual" refers to the general state of traffic during rush hour, indicating that heavy traffic is expected or normal.
Is "it" here a placeholder? What is usual for a Doric temple? The size? But where is the size mentioned earlier? If you refer to "larger," then it's an adjective. The pronoun "it" can't refer to the adjective.
Is "it" referring to "The Parthenon"? So, The Parthenon is larger than The Parthenon is usual for a Doric temple. Is "The Parthenon" a specific feature that is larger here and smaller there? No. Its a temple in itself. Wrong.