It is reported that a series of meetings ______ held about the best way to market the new product.

It is reported that 说明之后说的事是已经发生了的，所以要用过去式，排除B和D
meetings 是复数，所以排除A
剩下的were带进去，语法正确 ，因为主动式是hold meetings,变被动后就成了meetings were held 。

a series of meetings 先看这个主语，是复数哇？

It is reported that 再看这个句子是事情发生后的报道哇？属于过去式哇？

所以用were。

a series of meetings复数啊

C被动，复数，只有C

a series of 为单数,不能选C，要选A啊，而且是被动啊！

被动语态啊