Sentence Correction

Hi there,

I didn't succeed to get the right difference between A and B. OA is A


A number of linguists contend that all of the thousands of languages spoken by the world's five billion people can be traced back to a common root language.

(A) that all of the thousands of languages spoken by the world's five billion people can be traced
(B) that the world's five billion people speak thousands of languages of which all can be traced
(C) the world's five billion people speak thousands of languages which are all traceable
(D) all of the thousands of languages spoken by the world's five billion people to be traceable
(E) the ability to trace all of the thousands of languages that are spoken by the world's five billion people

In option B:

(B) that the world’s five billion people speak thousands of languages of which all can be traced

"of which all" is incorrect. When you use of which , a particular scenario or in this case, a language , can be traced , is correct.
For example, (B) that the world’s five billion people speak thousands of languages of which English can be traced .

Another option would be to use 'all of which' can be traced.So (B) should be written as
-- that the world’s five billion people speak thousands of languages all of which can be traced

I hope this helps.