That which cannot be understood

A. Illegible

B. Unintelligible

C. Unknown

D. Undecipherable

Answer: Option B

Solution(By Examveda Team)

One word substitute is Unintelligible.

Illegible: not clear enough to be read.
Unintelligible: impossible to understand.
Unknown: not known or familiar.
Undecipherable: (of speech or writing) not able to be read or understood.