Select the correct passive voice form for the given sentence.

Rashi was writing a letter to her husband.

A letter was being written by Rashi to her husband.

The correct passive voice form for the given sentence "Rashi was writing a letter to her husband" is A letter was being written by Rashi to her husband.

The active voice of the sentence is "Rashi was writing a letter to her husband." This sentence is in the past progressive tense, which is used to describe an action that was happening but was not finished. The passive voice of the sentence is formed by putting the subject of the active sentence after the verb "to be" and the present participle of the main verb. In this case, the subject of the active sentence is "letter" and the main verb is "write." The present participle of "write" is "writing." So, the passive form of the sentence is "A letter was being written by Rashi to her husband."

The other options are incorrect because:

• A letter were being written by Rashi to her husband." This sentence uses the plural form of the verb "were." The verb "were" is used for the plural subject "letters," but the subject of the sentence is "letter."
• A letter was been written by Rashi to her husband." This sentence uses the past perfect tense of the verb "to be." The past perfect tense is used to describe an action that happened in the past before another action happened. However, there is no other action mentioned in the sentence.
• A letter were been written by Rashi to her husband." This sentence uses the plural form of the verb "were" and the past perfect tense of the verb "to be." This is not a grammatically correct sentence.