That which cannot be understood
A
Illegible
B
Unintelligible
C
Unknown
D
Undecipherable
Correct Answer: Unintelligible
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.