What is the optimum answer to this GRE sentence-equivalence question?

The researcher noted that microbes, although _____, make up far more of the living protoplasm on Earth than all humans, animals and plants combined.

A. invisible

B. omnipresent

C. diminutive

D. ubiquitous

E. minuscule

F. ethereal

In my opinion, "diminutive" and "minuscule" are the appropriate words for the blank and the sentence makes sense.(Admittedly, microorganisms are extremely small, but they constitute a larger part of living protoplasm than all humans, animals and plants combined.)

However, that is at variance with the source of the sentence, an article of the New York Times(https://www.nytimes.com/2013/01/01/science/carl-woese-dies-discovered-lifes-third-domain.html), where the word "invisible" is used.

"Invisible" is unquestionably a proper choice, but does the word "ethereal", if filled in the blank, produce the same meaning with the original sentence?(Obviously, omnipresent and ubiquitous are ruled out.)

According to Merriam-Webster, "ethereal" means lacking material substance.(https://www.merriam-webster.com/dictionary/ethereal) I don't think microbes are devoid of material substance. They are just too tiny to be seen by the naked eye. Besides, "invisible" can refer to something that really exists: Using a telescope, Galileo discovered stars that were invisible to the naked eye.(https://www.ldoceonline.com/dictionary/invisible) Does the word "ethereal" really fit in well? If the best answers are "invisible" and "ethereal", how to exclude "diminutive" and "minuscule"?

Solution 1:

The researcher noted that microbes, although _____, make up far more of the living protoplasm on Earth than all humans, animals and plants combined.

The "far more...protoplasm" indicates a reference to either volume or mass and hence a straight forward interpretation along with the "although", indicates an adjective of smallness. Therefore, I believe that your assumptions of diminutive and minuscule seem about right. I can not think of a way to rule out those in absence of more surrounding context.

Language is a tool to convey information in a context, absent which, there may be ambiguities. Questions like these may not have one correct answer, whatever the rubric may say.