In the following question, a sentence has been given in Passive Voice. Out of the four alternatives suggested, select the one which best expresses the same idea in Active Voice: "These things have been left here by an unknown customer."

Introduction / Context:
This question asks you to convert a present perfect passive sentence into its active voice equivalent. The passive sentence is "These things have been left here by an unknown customer." You need to select the active version that preserves the same time reference, subject object roles, and meaning.

Given Data / Assumptions:

• Passive sentence: "These things have been left here by an unknown customer."
• Passive subject: "These things".
• Agent: "an unknown customer".
• Verb phrase: "have been left" (present perfect passive).
• Location adverb: "here".

Concept / Approach:
In present perfect passive, the pattern is "have/has been + past participle". To convert to active, we move the agent "an unknown customer" to the subject position, and change the verb to present perfect active: "has left". The object becomes "these things". Thus, the active equivalent should be "An unknown customer has left these things here." This keeps the present perfect sense (an action recently completed with present relevance) and retains all details.

Step-by-Step Solution:
Step 1: Identify the agent in the passive sentence: "by an unknown customer".Step 2: Move this agent to the subject position in the active sentence: "An unknown customer".Step 3: Recognise that "have been left" is present perfect passive for plural subject "these things".Step 4: Convert it to present perfect active with singular subject "An unknown customer": "has left".Step 5: Place "these things" as the object and keep the adverb "here": "An unknown customer has left these things here."

Verification / Alternative check:
Compare meanings: "These things have been left here by an unknown customer" and "An unknown customer has left these things here" both indicate a recent action of leaving items at the location, with the doer unknown by name but grammatically present. The present perfect tense suggests a present connection, such as the things still being there. This match confirms that option B is correct.

Why Other Options Are Wrong:

• Option A: "Leaves" uses simple present, suggesting a repeated or habitual action, which is not implied in the original sentence.
• Option C: Uses "have left" with singular subject "customer". This is a subject verb agreement error.
• Option D: "Has been left these things" is ungrammatical and reverses the roles incorrectly.
• Option E: "Left" uses simple past, losing the present perfect sense and present relevance of the action.

Common Pitfalls:
Students often mishandle present perfect when switching from passive to active by either changing tense or breaking subject verb agreement. Always match singular subjects with "has" and plural with "have", and keep the perfect aspect when needed. Also, make sure that the subject in the active sentence is the original agent after "by", and that the ex object retains its correct position and form.

Final Answer:
The correct active voice version is "An unknown customer has left these things here."