Who planted this tree here? The correct passive voice of this sentence is -

Who যুক্ত sentence কে passive করার structure -
By whom + auxiliary verb + subject + verb এর past participle form + extension (যদি থাকে).
সুতরাং, By whom was this tree planted here? সঠিক উত্তর হবে।
কিন্তু যদি Who had planted this tree here? থাকত তাহলে সঠিক উত্তর হত By whom had this tree planted here?