That which cannot be corrected

The correct answer is D. Incorrigible.

An incorrigible person is one who cannot be corrected or reformed. They are often stubborn and unwilling to change their ways.

The other options are:

• Unintelligible: not able to be understood.
• Indelible: impossible to erase or remove.
• Illegible: not able to be read.

All of these words describe something that is difficult or impossible to change, but only incorrigible describes a person who is impossible to correct or reform.