In the following question, a sentence has been given in one voice. Out of the four alternatives suggested, select the one which best expresses the same meaning in the other voice. 'John was arrested on a charge of murder, but for lack of evidence he was released.'

Introduction / Context:
This active passive voice question asks you to find the alternative that expresses the same meaning as the original sentence but switches the focus between subject and object. The original sentence is in passive voice and describes both John's arrest and his subsequent release due to lack of evidence. You must identify the option that correctly puts this information into an appropriate active voice form without changing the sense.

Given Data / Assumptions:
- Original sentence: 'John was arrested on a charge of murder, but for lack of evidence he was released.'
- The implied agent of the actions 'was arrested' and 'was released' is the police.
- Options include both active and passive variations, but only one properly converts to active voice for the main actions while preserving meaning and tense.

Concept / Approach:
In passive voice, the object of an action appears as the grammatical subject. Here, 'John' is the person affected by the actions 'was arrested' and 'was released'. To change to active voice, we must make the doer explicit, typically 'the police', and keep the same time reference (simple past). The best active version will therefore be: 'The police arrested John on a charge of murder, but for lack of evidence released him.' This sentence uses simple past active verbs 'arrested' and 'released' with 'the police' as subject and 'John/him' as object, while keeping the cause 'for lack of evidence' unchanged.

Step-by-Step Solution:
Step 1: Identify the two main actions in the original: 'was arrested' and 'was released'. Step 2: Recognise that the doer of both actions is the police, though not mentioned in the passive sentence. Step 3: Convert 'was arrested' to active: 'The police arrested John on a charge of murder.' Step 4: Convert 'he was released' to active: '(The police) released him for lack of evidence.' Step 5: Combine the two parts into a single sentence: 'The police arrested John on a charge of murder, but for lack of evidence released him.'

Verification / Alternative check:
Option (a) exactly matches the active reconstruction and preserves the contrast introduced by 'but', so it is correct. Option (b) keeps the sentence in passive voice and therefore does not switch the voice as required. Option (c) repeats the original mixed structure with both the police and passive clause, which is awkward and still partly passive. Option (d) changes the tense unnecessarily to past perfect ('had been arrested', 'had been released'), which alters the time relation and is not demanded by the original simple past sentence.

Why Other Options Are Wrong:
Option (b) is wrong because it remains entirely in passive voice; there is no change in the grammatical focus. Option (c) awkwardly mixes active and passive without improving clarity, and still leaves 'he was released' in passive voice. Option (d) is wrong because it uses past perfect, suggesting the arrest and release happened before some other past reference point, which is not indicated in the original sentence.

Common Pitfalls:
Students sometimes focus only on one part of a complex sentence when changing voice and forget to adjust the second action, or they unnecessarily change the tense in the process. When dealing with multiple clauses, always convert each main verb while preserving the original tense and logical connectors like 'but'. Also remember that adding the agent ('the police') is essential when you move from passive to active forms in such exam questions.

Final Answer:
The correct alternative is 'The police arrested John on a charge of murder, but for lack of evidence released him.'