the best way to process "as varied as" here is as an IDIOMATIC EXPRESSION.

here's a valid way to think about it: you can substitute "including" for "as varied as"; the sentence should still work.
these two don't have exactly the same meaning - i.e., "including" isn't the SAME as "as varied as" - but, if one of them is grammatically ok, then so is the other.

when you have a COMPARISON construction such as the one here ("as varied as..."), it must refer to whatever noun, noun phrase, or other type of construction IMMEDIATELY follows.
you cannot ignore intervening words. so if you have "as varied as those of...", then you must regard "those of..." as the object under comparison.

you'll find this rule pleasant, because it replaces difficult thinking with completely mechanical processes.

习惯用语-A as varied as B，A和B要平行，B用来解释、举例A
choice a, as varied as China, the Netherlands, and Hungary作为后置定语修饰countries，意思是诸如中国、荷兰、匈牙利等的国家
choice d, that are varied as China, the Netherlands, and Hungary 定语从句修饰countries, 意思是一定和中国、荷兰、匈牙利一模一样的那些国家

There's a very subtle difference in meaning, although the main issue is that of redundancy/wordiness. But consider the difference between these two sentences, which would illustrate the point:
(1) I get mail from cities as far away as Providence.
(2) I get mail from cities that are as far away as Providence.

If I'm in San Francisco, then sentence #2 means, strangely enough, that I get mail from cities that are all exactly 3,082 miles away (the distance from SF to Providence). Sentence #1 implies no such thing.

Similarly, the wrong answer (D) seems to imply that EACH country is somehow as 'varied' as EACH other country. That's not the intended meaning, which is that the SET of countries is varied.

A as varied as B A是概念，B用来解释A的；AB平行

as varied as =such as

国家之间的对比，可以排除BCE
AD之间对比
习惯用语-A as varied as B，A和B要平行，B用来解释、举例A
I get mail from cities as far away as New York. 我从像纽约那么远的城市收到一封邮件。强调形容词far
I get mail from cities that are as far away as New York.我从和纽约一样远的城市（都是xx公里远）收到一封邮件。that are在说明先行词的性质，和……一样远的先行词。

The phrase "as varied as" indicates some kind of comparison, so what are you comparing? What is it that is actually varying here? The noun 'countries" before the phrase and the three specific countries listed after the phrase. When you're comparing, you need the things you're comparing in parallel format.

So: "countries" on one side and "China, the Netherlands, and Hungary" on the other. Proper, parallel comparison.

Ron：
(1) I get mail from cities as far away as Providence.
(2) I get mail from cities that are as far away as Providence.
If I'm in San Francisco, then sentence #2 means, strangely enough, that I get mail from cities that are all exactly 3,082 miles away (the distance from SF to Providence). Sentence #1 implies no such thing.

习惯用语-A as varied as B，A和B要平行，B用来解释、举例A 同such as

AD差异：
A as varied as B = A 像 B 一样各式各样/种类繁多， 这里AB应平行

I get mail from cities as far away as New York. 我从像纽约那么远的城市收到一封邮件。强调形容词far
I get mail from cities that are as far away as New York.我从和纽约一样远的城市（都是xx公里远）收到一封邮件。that are在说明先行词的性质，和……一样远的先行词。

as varied as=such as，sth as varied as A, B and C。如：
species as varied as sheep, cattle, mink, deer and people.
Wrong：species that are as varied as sheep, cattle, mink, deer and people.表示一定是和后面列举的羊等一样的物种。

习惯用语-A as varied as B，A和B要平行，B用来解释、举例A

choice a, as varied as China, the Netherlands, and Hungary作为后置定语修饰countries，意思是诸如中国、荷兰、匈牙利等的国家
choice d, that are varied as China, the Netherlands, and Hungary 定语从句修饰countries, 意思是一定和中国、荷兰、匈牙利一模一样的那些国家

have effect on the base of countries, 后面跟定语修饰countries。as varied as sth介宾短语做定语。

把as varied as 看成such as，前面不加that

习惯用语-A as varied as B，A和B要平行，B用来解释、举例A。
varied
adj. 多变的; 各种各样的; 各不相同的; 形形色色的;

摘：a和d有细微的差别，a中countries包括China, the Netherlands, and Hungary，d中countries不包括China, the Netherlands, and Hungary

as varied as 后者用来解释前者，固定搭配