原句应该为：
I think (that)... trees were planted last spring.
I think (that)they planted.trees last spring.
就名词前的数量提问用how many；
do you think已经用了疑问句的形式,所以其后的从句不用再写成疑问语序,只要把疑问词提前即可,若想用主动形式答案B应改为they planted.
A,树是在过去被种,所以用过去被动式.
如果用B的话,应该改为：how many trees do you think they planted last spring.