Scientists at the state medical research facility have discovered a dormant virus, one that they believe is a type previously unknown to medical science.

A. that they believe is

B. that they believe it to be

C. they believe that it is of

D. they believe that is

E. that they believe to be of

This question is based on Idiomatic Usage and Construction.

A vertical scan of the options shows us that the options give us a choice between whether to use ‘that’ or not. The relative pronoun ‘that’ should ideally be used in this sentence as it makes the clause that follows more specific. 

Neither Options C nor D contains the relative pronoun ‘that’. 

However, there are stronger reasons to rule out these options. In Option C, the pronoun ‘it’ is redundant. The indefinite pronoun ‘one’ already refers to “the virus”. Since, there is no need for another pronoun to refer to the same antecedent, Option C can be eliminated.

The placement of the pronoun ‘that’ in Option D makes the pronoun ‘that’ ambiguous. If it is placed at the beginning of the option, it acts as a relative pronoun that connects the clause to the rest of the sentence. However, placed as it is in this option, the pronoun does not have a clear antecedent. So, Option D can be eliminated.

In Option B also, the pronoun ‘it’ is redundant. So, Option B can be eliminated. 

Between Options A and E, Option E is better because the preposition ‘of’ must be used with the noun “a type”. So, Option A can be eliminated.

Therefore, E is the most appropriate option.