选择A.
该句的意思是大部分被邀请参加贝蒂生日宴会的客人是她的同学.注意：这里用到了定语从句,而定语从句又是对Most of the guests在起限定作用.邀请的主语是Betty,宾语是guests,guests是是被邀请的,所以原句可以这么理解,Most of the guests ___(who were )invited___ to Betty's birthday party were her classmates.定语从句的主语谓语可以省略,所以省略了就是原文的Most of the guests ___ invited___ to Betty's birthday party were her classmates.
had been invited 和were invited 的区别：
had been invited 强调被邀请的过去完成的状态,而were invited 则表示被邀请的过去的状态.
是时态的不同,导致选用的形式不同.