In the following error-spotting question, identify the part of the sentence that contains a grammatical error. One (A) / should keep (B) / his word. (C) / No Error (D).

Introduction / Context:
Error-spotting questions check your understanding of standard grammatical conventions and modern usage in English. The sentence given is “One (A) / should keep (B) / his word. (C) / No Error (D).” You must decide which part contains an error, if any. This particular question tests your knowledge of pronoun agreement when the indefinite pronoun “one” is used as the subject.

Given Data / Assumptions:

• Part (A): “One”
• Part (B): “should keep”
• Part (C): “his word.”
• Part (D): “No Error” (indicates that the sentence may be error-free)
• The sentence is meant to express a general rule or moral principle.

Concept / Approach:
In formal and traditional English, when we use the indefinite pronoun “one” to talk about people in general, the corresponding possessive pronoun should also be “one's”, not “his” or “her”. The standard pattern is “one should keep one's word”, “one should do one's duty”. Therefore, even though “his word” is commonly heard in casual speech, in strict grammatical and exam contexts it is considered incorrect when the subject is “one”. The error is in the mismatch between “one” and “his”.

Step-by-Step Solution:
Step 1: Identify the subject in Part (A): “One”. It refers to any person in general. Step 2: Look at the verb phrase in Part (B): “should keep”, which is correct as a modal construction expressing advice or obligation. Step 3: Examine the pronoun in Part (C): “his word”. This pronoun should agree with the subject “one”. Step 4: Recall the rule: when “one” is used, the possessive pronoun should be “one's”. Step 5: Conclude that the corrected sentence should be “One should keep one's word.” Thus, the error lies in Part (C).

Verification / Alternative check:
To verify, consider similar sentences: “One must do one's duty”, “One should be careful about one's actions.” In all these, standard grammar books recommend repeating “one” in the possessive form “one's” instead of switching to “his” or “her”. While modern informal English sometimes uses “their” for gender neutrality, exam-standard usage with “one” still prefers “one's”. This confirms that “his word” does not agree with “One” and is therefore the erroneous part.

Why Other Options Are Wrong:
“Part (A)” contains “One”, which is correct as the subject of a general statement.
“Part (B)” has “should keep”, a correct modal verb phrase expressing obligation.
“Part (D) No error” is not correct, because we have already identified a specific grammatical mismatch in Part (C).
“Whole sentence” as an error label is incorrect, because only the pronoun needs correction; the rest of the sentence is fine.

Common Pitfalls:
Many students overlook this finer point of pronoun agreement and accept “his word” as natural sounding, especially if they have heard it frequently. Another common mistake is to forget that competitive exams often follow conservative grammar rules, emphasizing consistent reference. To avoid such pitfalls, remember that when you start a sentence with “one”, you should maintain that pattern: “one … one's”. Switching to “his”, “her”, or “their” is usually penalized in formal tests.

Final Answer:
The grammatical error is in Part (C), which should read “one's word” instead of “his word”.