The active voice sentence is "I have kept all your old letters." Choose the option that best expresses this sentence in the passive voice.

Introduction / Context:
This question tests how to convert present perfect tense in active voice into present perfect passive voice. The original sentence emphasises the speaker as the person who has preserved all the letters. In the passive version, the emphasis shifts to the letters themselves, while the tense and basic meaning remain the same.

Given Data / Assumptions:

• Active sentence: I have kept all your old letters.
• Subject: I.
• Object: all your old letters.
• Verb phrase: have kept (present perfect).
• The action of keeping is complete at the time of speaking, and the result is relevant to the present.

Concept / Approach:
In present perfect passive voice, the structure is: subject (previous object) + has/have been + past participle. Here, "all your old letters" is the object and becomes the subject of the passive sentence. The verb phrase "have kept" then becomes "have been kept." We also keep "by me" as the agent phrase to indicate who has done the action, although this is sometimes omitted in natural language when the agent is obvious or unimportant.

Step-by-Step Solution:

Step 1: Identify the object "all your old letters." This will become the subject in the passive form. Step 2: Recognise that the verb "have kept" is present perfect with a plural object, so the passive auxiliary must be "have been" plus the past participle "kept." Step 3: Retain the phrase "by me" to show that the speaker is the one who has kept the letters. Step 4: Combine the elements to form the passive sentence: "All your old letters have been kept by me."

Verification / Alternative check:
The passive sentence "All your old letters have been kept by me" correctly uses present perfect passive for a plural subject. It keeps the same time reference and result meaning as the original active sentence. The focus now rests on the letters, which is appropriate for many formal contexts where the state of the objects is more important than the person acting.

Why Other Options Are Wrong:

Option B: "Keeping of all your old letters has been done by me." is grammatically heavy and unnatural; it uses an unnecessary nominalisation and also changes the auxiliary to "has been" with a singular subject "keeping." Option C: "Your letters which have become old would be kept by me." uses "would be kept," which introduces a conditional meaning rather than a completed present result. Option D: "I keep your letters which have become old." is a simple present active sentence and does not reflect present perfect or passive voice. Option E: "All your old letters were being kept by me." uses past continuous passive, suggesting an ongoing past action rather than a completed action relevant to the present.

Common Pitfalls:
One common error is to forget "been" in perfect passive forms, writing "have kept" instead of "have been kept." Another pitfall is shifting the tense unintentionally to simple past, past continuous, or another form, which changes the time reference. Learners should always check whether the original sentence uses present perfect, and then match it with the correct passive pattern "has/have been + past participle."

Final Answer:
The correct passive voice sentence is All your old letters have been kept by me.