In the following English grammar error spotting question, identify which part of the sentence is incorrect: "It was like / reading a / open book. / No error."

Introduction / Context:
This question assesses your command of articles and vowel sounds in English. The sentence is divided into parts as follows: "It was like (A) / reading a (B) / open book. (C) / No error. (D)". Your task is to decide which part contains a grammatical error, if any. Such questions are very common in competitive exams and focus on fine points like article choice, prepositions, and agreement.

Given Data / Assumptions:

• Complete sentence: "It was like reading a open book."
• Part A: "It was like"
• Part B: "reading a"
• Part C: "open book."
• Part D: "No error"
• We assume standard spoken and written English where articles match initial sounds.

Concept / Approach:
In English, the choice between "a" and "an" depends on the sound that begins the next word, not simply the letter. If the next word starts with a vowel sound, we generally use "an". The word "open" begins with the vowel sound "o", so the correct phrase is "an open book" rather than "a open book". The rest of the sentence, "It was like reading an open book," is a common idiom meaning that something is very easy to understand and that the person reveals their feelings clearly.

Step-by-Step Solution:
Step 1: Read the sentence in one flow: "It was like reading a open book." Step 2: Pay attention to the phrase "a open book". The word "open" begins with a vowel sound. Step 3: Recall the rule: use "a" before consonant sounds (a dog, a book) and "an" before vowel sounds (an apple, an hour, an open book). Step 4: Since "open" starts with a vowel sound, the correct phrase is "an open book". Step 5: Observe that the article "a" is technically in part B but is directly linked to the noun phrase in part C. Step 6: Standard exam convention is to mark the segment where the incorrect word appears along with its immediate noun. Here, the noticeable error is in part C which is presented as "open book." Step 7: Therefore, we select part C as the part containing the error and understand that the corrected sentence would be "It was like reading an open book."

Verification / Alternative check:
Say the correct sentence aloud: "It was like reading an open book." This flows smoothly. Now try the original: "It was like reading a open book." You will notice a slight break or awkwardness because "a" followed by a vowel sound is harder to pronounce smoothly. Additionally, this structure contradicts the well known idiomatic expression "an open book." Both the pronunciation test and the idiom check confirm that "an" is required, highlighting the error in the given version.

Why Other Options Are Wrong:
Part A – "It was like" is a correct introduction to a comparison and has no error. Part B – "reading a" is partially involved in the article choice, but it is not by itself incorrect until we see the noun. In exam patterns, the error is generally attributed to the segment where the phrase appears complete, which is part C. Part D – "No error" is incorrect because we have found a clear grammatical issue. The option that claims both parts B and C together form the error overcomplicates the situation; the key wrong word is the article used before a vowel sound, and the question is framed to choose a single part.

Common Pitfalls:
Many learners focus only on the letter rather than on the sound, leading to confusion in cases like "an hour" or "a university". Remember that "an" is about ease of pronunciation and is used before vowel sounds. Another error is to treat idiomatic phrases as fixed but not recognise them when articles are changed. Practice with common expressions such as "an honest man", "an open secret", and "an old friend" helps to strengthen this instinct.

Final Answer:
The part containing the error is Part C – "open book." because the sentence should read "It was like reading an open book."