The correct answer is (b) A letter is being written by me.

Explanation:

- Active Voice: The sentence "I am writing a letter" is in the active voice because the subject (I) is performing the action (writing) on the object (a letter).

- Passive Voice: To change the sentence into the passive voice, we need to make the object (a letter) the subject of the sentence and the subject (I) the object of the preposition "by." We also need to use the appropriate form of the verb "be" and the past participle of the verb "write."

- Option (a): The letter is written by I.
This option is incorrect because it uses the incorrect word order. In passive voice sentences, the subject comes after the verb "be" and before the past participle.

- Option (b): A letter is being written by me.
This option is correct because it follows the correct word order and uses the present continuous form of the verb "be" (is being) to indicate an ongoing action. The subject (a letter) is performing the action (being written) and the object (me) is introduced with the preposition "by."

- Option (c): A letter is written by me.
This option is also correct, but it does not indicate that the action is happening in the present. It simply states that a letter is written by me without specifying when the action took place.

- Option (d): The letter was being written by me.
This option is incorrect because it uses the past tense form of the verb "be" (was being), indicating that the action was happening in the past. However, the original sentence is in the present tense, so the correct option should reflect the same tense.

In summary, the correct answer is option (b) "A letter is being written by me" because it accurately changes the active voice sentence "I am writing a letter" into the passive voice, maintaining the correct word order and tense.