Step 1: Understanding the Question:
This question asks us to identify the part of the sentence that contains a grammatical error.
The complete sentence reads: ``They had prepared meticulously for the project, but unforeseen technical issues disrupted them plan.''
Step 2: Key Concepts and Grammar Rules:
The error lies in the choice of pronoun in the final clause: ``disrupted them plan''.
In English, ``them'' is an objective personal pronoun used as the object of a verb or preposition (e.g., ``disrupted them'').
However, when modifying a noun (like ``plan'') to show ownership or possession, we must use a possessive adjective (determiner) instead.
The possessive adjective corresponding to ``They'' is ``their''.
Step 3: Detailed Explanation:
• Part (A): ``They had prepared meticulously'' is grammatically correct. It uses the past perfect tense (had + V3) and the adverb ``meticulously'' properly modifies the verb ``prepared''.
• Part (B): ``for the project, but unforeseen'' is correct. The preposition ``for'' matches the purpose, and ``unforeseen'' is a suitable adjective modifying ``technical issues''.
• Part (C): ``technical issues disrupted them plan'' contains the error. The phrase ``them plan'' is incorrect because ``them'' is an objective pronoun. It must be replaced by the possessive adjective ``their'' to form ``their plan''.
Step 4: Final Answer:
Since the error is located in Part (C), Option (C) is the correct choice.