Error spotting: in the following question, one part of the sentence may have an error. Find out which part of the sentence has an error. If the sentence is free from error, choose the option marked No error. I was (A) / at loss (B) / and did not know what to do (C) / No error (D)

Introduction / Context:
This question examines your familiarity with common English idiomatic expressions, specifically at a loss. When describing confusion or uncertainty, English uses a fixed phrase that must include the article a. Leaving it out makes the phrase ungrammatical or at least very unnatural in standard usage.

Given Data / Assumptions:

• Sentence: I was at loss and did not know what to do.
• Part (A): I was.
• Part (B): at loss.
• Part (C): and did not know what to do.
• Part (D): No error.
• Intended meaning: I was confused or puzzled and did not know what action to take.

Concept / Approach:
The standard expression in English is at a loss, meaning uncertain, puzzled, or not knowing what to do. The article a is an essential part of the idiom. Therefore, at loss is incorrect. The rest of the sentence I was and did not know what to do is correct. As a result, the error must be in part (B), where at loss should be corrected to at a loss.

Step-by-Step Solution:
Step 1: Identify the common idiom that the sentence appears to be using. Step 2: Recall that the correct form is at a loss, not at loss. Step 3: Check parts (A) and (C) for any tense or structure errors; both are fine. Step 4: Conclude that part (B) is where the error lies because it omits the necessary article a.

Verification / Alternative check:
Test the sentence mentally after correction: I was at a loss and did not know what to do. This sounds perfectly natural and is commonly used in spoken and written English. Checking phrasebooks or dictionaries of idioms confirms that at a loss is the recognised expression meaning perplexed or uncertain. There is no variation at loss that carries the same idiomatic sense.

Why Other Options Are Wrong:

• Part (A): I was is correct as the subject and auxiliary verb for a past state.
• Part (C): and did not know what to do is correctly formed and logically follows from being at a loss.
• Part (D): No error is incorrect because we have identified a clear idiom error in part (B).

Common Pitfalls:
Learners sometimes forget the small but important article in idiomatic phrases such as in a hurry, in a fix, or at a loss. They re create them as in hurry or at loss, which sound odd to native speakers. Error spotting questions frequently target these subtle forms. To avoid such mistakes, try to memorise idioms as complete units rather than as separate words.

Final Answer:
Correct option: B.