The correct passive of 'Rana was writing a letter.' is -

Active Voice: 'Rana was writing a letter.' 
Passive Voice: A letter was being written by Rana. 

• Past Continuous Tense এর Active Voice কে Passive Voice এ পরিবর্তন করার নিয়ম: 
- Object টি subject হয়, 
+ was being/were being বসে, 
+ মূল verb এর past participle বসে, 
+ by বসে, 
+ Subject টির objective form বসে। 

• Structure:
Object টির subjective form + was being/were being + মূল verb এর past participle + by + subject এর objective form.