答案为 is going to be
解析如下
there be句型
there be 的将来形式为 there will be.跟 there is/are going to be
鉴于有4个空
There is going to be a party at her home next Saturday evening.
have在这行为动词（表示有的意思）,不能够和there 搭配.
另there也不是名词,没办法单独当个主语来搭配have.
所以这同义句 答案的改动只在 have 到 be