C
句意：
既然已经制定了计划,现在所要做的就是怎样执行它.
分析：
A项been多余；
B项只能做谓语,用于句子或从句中.但本题逗号前面不是从句,因为它前后没有相关的连词.
D项错在用了主动式,而本题中的plan与made是动宾关系,也就是被动关系.
the plan having been made可以转换成从句：
Now that the plan had been made.
或者这样转换：
The plan had been made,so what is to be done now is how to carry it out.
不明白的再问哟!请记得采纳,多谢!