In the following sentence, one part may contain an error. Identify the part with the error; if the sentence is correct, select "No error": A. He was not allowed B. for the teacher C. to read in a low voice D. no error.

Introduction / Context:
Error spotting items often focus on prepositions and agents in passive constructions. In English, phrases like was allowed are frequently followed by the preposition by when introducing the person who grants permission or imposes restriction. Recognising the difference between for and by in this role is essential for accurate sentence construction.

Given Data / Assumptions:
The sentence is divided into parts: A. He was not allowed B. for the teacher C. to read in a low voice D. no error. Part B uses the phrase for the teacher as if it introduced the agent of the action. The intended meaning is that the teacher did not allow him to read in a low voice. We assume a standard passive structure where the person denying permission is mentioned.

Concept / Approach:
When we use a passive verb like was allowed or was punished and wish to mention the person who performs the action or makes the decision, we introduce that person with by, not for. The structure is was not allowed by the teacher, which correctly identifies the teacher as the agent. The preposition for usually indicates benefit or purpose, as in This is for the teacher, and does not fit the agent role in a passive sentence. Therefore, the preposition in part B is grammatically incorrect.

Step-by-Step Solution:
Step 1: Identify the passive verb phrase was not allowed in part A.
Step 2: Recognise that the next phrase for the teacher is supposed to name the person who did not allow him.
Step 3: Recall the rule that the agent in a passive sentence is introduced by the preposition by.
Step 4: Replace for with by to form the correct phrase by the teacher.
Step 5: Conclude that the error lies in part B, where the wrong preposition for was used.

Verification / Alternative check:
The corrected sentence reads: He was not allowed by the teacher to read in a low voice. If we change this to the active voice, it becomes: The teacher did not allow him to read in a low voice. Here, the teacher is clearly the doer of the action, and in the passive version the preposition by is normal. The expression for the teacher, by contrast, suggests that something is intended for the teacher, not that the teacher is the one who allows or forbids. This comparison confirms that part B is wrong and should be corrected to by the teacher.

Why Other Options Are Wrong:
Part A, He was not allowed, is a correct passive form. Part C, to read in a low voice, is a proper infinitive phrase describing what action is not allowed. Part D, no error, is only a label and does not form part of the sentence itself. Since only the preposition in part B is inappropriate, option B is the right choice as the location of the grammatical error.

Common Pitfalls:
Learners sometimes confuse by and for, especially when translating directly from their first language. They may also omit the agent in passive constructions altogether and thus become unsure which preposition to use when the agent is added. A good practice is to mentally convert passive sentences to active form and identify the subject clearly. Then, when turning back to the passive, use by to introduce that subject if it must be mentioned.

Final Answer:
The error is in part B; the phrase should be by the teacher, not for the teacher.