Jawaban:

A letter is being written by us now.

Penjelasan:

Dalam kalimat passive, harus diingat bahwa verb nya harus selalu verb-3, tidak peduli kalimatnya dalam tense mana. Mau present, perfect, past, future dll, verb harus dalam bentuk verb-3

Verb di kalimat ini adalah: write.

Kalimat ini adalah kalimat dalam present continuous tense, jadi verb-ing => writing

Dalam present continuous tense, untuk membuat kalimat passive kita butuh auxilliary being

Strukturnya:

Subjek + to be (is/am/are) + being + verb 3 + by objek + keterangan dst

Contoh lain:

(Active) He is throwing the ball

(Passive) The ball is being thrown by him

(Active): My parents are leaving the airport

(Passive) The airport is being left by my parents

(Active) I am boiling the eggs

(PAssive) The eggs are being boiled by me

Note: kenapa di sini pakai are?Karena subjek kalimatnyaThe eggs, jamak/plural.

Semoga bermanfaat.