the papers were given out to everyone

您好，意思是：论文给出了每个人。
1）once在此题中意思是“一旦”,引导条件状语从句.但是该从句却没有主语,说明它采用了省略的语法.
2）状语从句的省略规定如下：如果状语从句的主语是it或与主句主语相同,且状语从句的谓语动词以be动词开头,则可以把状语从句的主语和be动词一起省略.
该语法现象在高中阶段常见常考.
即：Once seated = Once everyone is seated.
3）如果选C,则变成Once everyone is being seated,“一旦每个人正在就座”,句意不通。