All of the letters in the Cyrillic alphabet, one of which is borrowed from either the Greek or Latin alphabet, represents specific sounds in spoken Russian.

all of the letters represents---> s-v error.. the verb should be represent so A and D out
one of which---->wrong idiom... it should change to each of which so C and D out
E wins

in the fist step, you should separate each clauses.
1st clause:All of the letters in the Cyrillic alphabet, one of
2nd clause:which is borrowed from either the Greek or Latin alphabet,
(cont. of 1st clause)represents specific sounds in spoken Russian.

in each clause we should have subject and verb.

in the 1st clause we have : "all of the letters" as the subject but no verb.
in the second clause we have : "which" as the subject that refers to "one" and "is borrowed" as the verb
in cont. of 1st clause we have "represents" that is verb. so, it is referring to the first subject.

After reading all of the posts, I want to contribute my explanations so that it will be much clearer for the newcomers to understand this simple but tricky question.


I. First, look at all "statements" in the sentence: Two commas separate the sentence into 3 parts:
- First part: "All of the letters in the Cyrillic alphabet": A noun phrase => So think: to fulfill a CLAUSE (or sentence), we may need verb. I say "may" because if this noun phrase is a modifier for something else (for the real subject of the sentence), then it requires no verb. At that time, by itself, it is enough.
- Second part: "one of which is borrowed from either the Greek or Latin alphabet": a non-defining relative clause. To those who don't understand what is a defining or non-defining relative clause, just understand in simple way: it is a clause which modifies certain things, different in that "defining" means the clause is necessary, without such information, the sentence cannot be understood fully (NO COMMA required), and non-defining means the clause just provide more information and can be deducted (separated by AT LEAST one comma). More information will be found in any grammar text book => So think: the second part is a non-defining clause, so it MUST BE a modifier. Ask yourself: it modifies what? Doubtless, it modifies the noun phrase in the first part.
- From that reasoning, the third part will HAVE TO contain a verb (represent) influenced by the subject (the noun phrase) in the first part, which means the word "one of which" in the second part DOES NOT influence the grammatical point of the word "represent" in the third part.
- Accordingly, choice A and D are omitted.


II. Now, look at C, what does the phrase "each of which of those" mean? It means... nonsense in a redundant way. For those who just cannot understand its redundancy, ask yourself what NOUN the word "which" and the word "those" represent! Then, you see the existence of the both two words are funnily redundantly.


III. So now, the fatal problem lies in choice B and D: the difference between "one of which" and "each of which". Because grammatical point is right for both terms, to understand the difference, we have to lie in the meaning:
- From the basic understanding of relative clause above, ask your self: what does "one of which" and "each of which" mean? More specifically, what does "which" represent? Surely, which substitutes for the WHOLE noun phrase "All of the letters in the Cyrillic alphabet". Thus, by meaning, it is clear that when we say "one/each of which", we mention about the INDIVIDUAL letters (and all of these letters are in the Cyrillic alphabet). Then, the crucial importance lies in the difference between the "individuality" of the word "each of which" and "one of which". Consider the following examples:
- One of the students is sick today => What do you mean? Ah, you mean there is A "random" student who is sick. Only ONE student.
- Each of the students is sick today => What do you mean? Ah, you mean if the class has 100 students, all of them are sick (by counting one by one: one is sick, two is sick, three is sick,... my god all are sick)

- Now it is extremely easy to understand our sentence:
- If we say "one of which", meaning "one of the letters", there is one and only one random letter mentioned. If you argue that it is ok because non-defining clause can provide unnecessary information (and can be deducted), you are wrong because such information, albeit unnecessary, MUST BE DIRECTLY relevant IN MEANING to the noun it modifies. To understand in the most raw common sense, the meaning of the two sentences containing "one of which" and "each of which" is illustrated in these two sentences:
- For "one of which": All of the letters in the Cyrillic alphabet, [the Russian letter is borrowed from either the Greek or Latin alphabet], represent.... Now, the Russian letter is the raw transformation of the meaning of the word "one of which". Look at the meaning, you must be laughing at its nonsence. But look at "each of which":
- For "each of which": All of the letters in the Cyrillic alphabet, [the Russian letter, the American letter, the Australian letter,... all are borrowed from either the Greek or Latin alphabet], represent...



Now I hope you fully understand this tricky question.