One-word substitute: Handwriting that cannot be easily read:

Correct Answer: A. Illegible 

The term “illegible” is used to describe handwriting that cannot be easily read or deciphered. Illegible handwriting is difficult to understand or make out due to factors such as poor penmanship, messy writing, or unclear letter formation. When something is illegible, it is challenging or impossible for others to read or comprehend the written text.