Step 1: Understanding the question.
The question asks for the word that best substitutes the phrase "That which cannot be corrected." The correct word must mean something that cannot be improved or changed.
Step 2: Analyzing the options.
(A) Unintelligible: This means something that is hard to understand, but it does not fit the meaning of "cannot be corrected."
(B) Indelible: This means something that cannot be erased or removed, such as a mark or memory, but it does not mean "cannot be corrected."
(C) Illegible: This refers to something that is hard to read, but it does not mean "cannot be corrected."
(D) Incorrigible: Correct — This word means something that cannot be corrected, reformed, or improved, which fits the definition of the phrase.
Step 3: Conclusion.
The correct answer is (D) Incorrigible, as it means "cannot be corrected."