In this example that is unnecessary, the pronoun one adequately refers to a microscopic plant parasite. Because the sentence refers to something that existed a long time ago, but has only been documented of late, to be is more appropriate than is. Is is in the present tense, and it is best to find an answer choice that encompasses both the past and present - the infinitive to be does this better. After eliminating that and is, we are left with choice E
. This question has an assumption,(they assume), in effect it is not a fact. If we follow this reasoning through, to be a type is stating fact, whereas to be of a type is not quite as emphatic, it is an assumption.
"Start By Doing What Is Necessary ,Then What Is Possible & Suddenly You Will Realise That You Are Doing The Impossible"