From the given options, select the one that rightly substitutes the given expression:

Incapable of being read

Illegible

The word that rightly substitutes the given expression is illegible.

• Indelible means "impossible to remove or erase."
• Illegible means "impossible to read."
• Incorrigible means "impossible to correct or reform."
• Inexplicable means "impossible to explain."

Illegible is the best fit because it conveys the idea of something that cannot be read, either because the writing is too small or faint, or because the letters are not formed correctly. The word comes from the Latin word "illegibilis," meaning "not able to be read."