Concept:
This question tests the principle of Subject-Verb Agreement. In English grammar, the verb must always agree with the true subject of the sentence in terms of number (singular or plural).
Step 1: Identify the true Subject.
In part (A), we find the subject: "The diplomatic actions".
Note that "actions" is a plural noun. The phrase "against Pakistan taken by India’s Cabinet Committee on Security" is a modifier providing more detail about those actions, but the core subject remains "actions."
Step 2: Identify the Verb and check for agreement.
In part (C), the verb used is "is".
• Subject: The diplomatic actions (Plural)
• Verb: is (Singular)
There is a clear mismatch. A plural subject requires a plural verb.
Step 3: Determining the correction.
The correct verb should be "are".
The corrected sentence would read: "The diplomatic actions... are a measured set of responses...". Therefore, part (C) contains the grammatical error.
Final Answer: Option C