THAT

[karthikpandian19]

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

[Bill@VeritasPrep]

We definitely need "that" because you can't contend people (C), all of the thousands of languages (D), or an ability (E). B uses the awkward construction "of which all", leaving us with A.

[GmatKiss]

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

[karthikpandian19]

OA is A