In the following error spotting question, read the labelled sentence I think so (A) / you have taken (B) / the right decision. (C) / No Error (D) and choose the part (A), (B), (C) or (D) that contains the mistake in standard English usage.

Introduction / Context:
This question examines correct idiomatic usage in English, particularly in how the phrase I think is used before a that clause. The sentence is broken into four labelled parts, and you must decide which part contains a usage error. The speaker wants to say that in their opinion, someone has taken the right decision. The problem lies not in grammar at the level of verb forms, but in a small, unnecessary word that makes the expression unidiomatic in this context, which is a common type of exam trap in error spotting questions.

Given Data / Assumptions:

• Part (A): I think so
• Part (B): you have taken
• Part (C): the right decision.
• Part (D): No Error.
• The intended meaning is I think you have taken the right decision.

Concept / Approach:
The key concept is the difference between I think and I think so in English. The phrase I think so is used when responding to a yes or no question, for example, Will it rain today? I think so. However, when you introduce a full clause explaining your opinion, such as you have taken the right decision, we normally say I think you have taken the right decision and do not insert so between think and the clause. Therefore, the use of so in this sentence is incorrect and unidiomatic. The rest of the sentence, including the present perfect verb have taken and the noun phrase the right decision, is grammatically correct.

Step-by-Step Solution:

Verification / Alternative check:
To verify, rewrite the sentence in a natural way: I think you have taken the right decision. This version is idiomatic and conveys a clear opinion. If someone had asked, Have I taken the right decision?, an appropriate short answer would be I think so, but that would stand alone, not be followed by you have taken the right decision. Thus, combining I think so with a full clause is redundant and unnatural. This confirms that the sentence should start with I think, without so, and that part (A) as given in the question is incorrect usage.

Why Other Options Are Wrong:
Part (B), you have taken, is correct present perfect construction and fits well with the idea of a decision already made. There is no need to change the tense or the auxiliary. Part (C), the right decision, is a standard noun phrase, and both the article and adjective are used correctly. Part (D), No Error, cannot be the correct choice because we have clearly identified a usage issue in part (A). Therefore, only option A correctly marks the segment that needs to be improved to achieve natural English expression.

Common Pitfalls:
A frequent mistake is to overlook small function words like so and focus only on verbs or nouns. Some learners wrongly try to change the tense in have taken or alter the phrase the right decision, even though both are correct. It is important in error spotting to consider not just grammar rules but also idiomatic patterns of everyday English. When you see I think followed by a complete clause, remember that so is not used in such structures and is reserved for short answers to yes or no questions. Training your ear for natural combinations of common phrases can greatly improve performance in these items.

Final Answer:
The incorrect part is (A), so the correct answer is A.