The correct passive voice form for the given sentence is:

☀ A letter was being written by Sudha to her husband.

So, the correct passive form is: "A letter was being written by Sudha to her husband."

The other options and their explanations:

☀ A letter were been written by Sudha to her husband.

• This option contains a grammatical error. The subject "letter" is singular, so it should be "was" instead of "were."

☀ A letter were being written by Sudha to her husband.

• This option also contains a grammatical error. The subject "letter" is singular, so it should be "was" instead of "were." Additionally, the correct past participle form is "been" instead of "being."

☀ A letter was been written by Sudha to her husband.

• This option contains a grammatical error. The correct form is "A letter was written by Sudha to her husband." The use of "been" is incorrect in this context.