Select the most appropriate option to substitute the underlined segment in the given sentence: There’s little evidence available of to what exactly makes this more threatening.

Correct option is C

The correct answer is (c) as to. The phrase “evidence available of to what exactly…” is grammatically incorrect and awkward. The correct phrase should be "evidence available as to", which is a standard and grammatically correct construction used to introduce the subject or content of the evidence, opinion, or information.
“As to” means "regarding" or "concerning", and fits naturally in formal contexts like research, reports, or analytical discussion. The corrected sentence becomes: “There’s little evidence available as to what exactly makes this more threatening.” This sentence now reads smoothly and conveys the intended meaning without ambiguity.