In this English grammar question, identify which part of the sentence is incorrect: "All of them speak good english."

Introduction / Context:
This question examines knowledge of capitalization rules in English, specifically for proper nouns and language names. The sentence is "All of them speak good english." Although the grammar of the sentence is otherwise correct, one word is not written according to standard conventions. Recognising such issues is important for formal writing, academic examinations, and professional communication.

Given Data / Assumptions:

• The sentence comments on language ability: "All of them speak good english."
• Segment (a) is "All of them".
• Segment (b) is "speak".
• Segment (c) is "good english".
• Segment (d) is "No error".
• We assume normal capitalization rules where names of languages start with capital letters.

Concept / Approach:
Names of languages, nationalities, and proper nouns always begin with a capital letter in English. Words such as English, Hindi, French, German, and Spanish must be capitalized. In the sentence, the word "english" appears in lower case, which is incorrect in standard writing. The rest of the sentence structure is acceptable, as "All of them speak good English" is a perfectly natural sentence. Therefore, the error lies in the third segment where capitalization is missing.

Step-by-Step Solution:
Step 1: Examine segment (a) "All of them". This phrase is grammatically correct and acts as the subject of the sentence. Step 2: Examine segment (b) "speak". This is a correct present tense verb that agrees with the plural subject. Step 3: Examine segment (c) "good english". The adjective "good" is fine, but check the noun "english". Step 4: Recall that language names must begin with a capital letter, so "english" should be "English". Step 5: Recognise that this missing capitalization is a standard error often tested in exams. Step 6: Conclude that segment (c) is the incorrect part of the sentence.

Verification / Alternative check:
Compare with similar sentences: "She speaks fluent English", "They are learning French", and "He knows some Japanese." In every case, the language name uses an initial capital letter. If written in lower case, it would be considered wrong in formal contexts. Applying this rule to the given sentence shows that "english" must be corrected to "English". Therefore, the problem is clearly in segment (c), and no other corrections are required.

Why Other Options Are Wrong:
Option (a) "All of them" is a correct subject phrase for a plural group. Option (b) "speak" correctly matches the plural subject in the present tense. Option (d) "No error" is not suitable because the capitalization mistake in segment (c) is real and important.

Common Pitfalls:
In informal texting or online communication, people often write everything in lower case, including proper nouns. This habit can carry over into exam answers, causing candidates to miss capitalization errors. Remember that formal English keeps strict capitalization rules for names of people, places, and languages. Making a conscious effort to correct such habits in practice exercises will greatly improve performance in written examinations.

Final Answer:
The incorrect part is option (c) "good english", where "english" should be capitalized as "English". Therefore, the correct answer is "good english".