选A.
Who do you think he would have write a letter of congratulation to her teacher?
“你认为他会让谁写一封祝贺信给她的老师?”
who在句子中作have的宾语.
have sb.do sth.“让某人做某事”,sb.是do的发出者；
例句：I'll have someone repair the bike for you.我会让人为你修理自行车的.
have sb./sth.done sth.“让某人/某事被做”,sb./sth.与done是被动关系；
例句：The patient is going to have his temperature taken.这个病人准备请 / 叫 / 让人量体温.