There ()a meeting tomorrow afternoon. A will B will going to be C is going to be D will go to be He

选C. is going to be.

A项差be,不合语法；B和D是乱搭配，不符合表达习惯。

相关知识点参考：
be going to与will两者都可表示将要发生的事、将要去做某事，但它们有如下几点区别：

1. be going to 表示近期、眼下就要发生的事情，will 表示的将来时间则较远一些，如：

He is going to write a letter tonight.

He will write a book one day.

2. be going to 表示根据主观判断将来肯定发生的事情，will表示客观上将来势必发生的事情。

He is seriously ill. He is going to die.

He will be twenty years old.

3. be going to 含有“计划，准备”的意思，而 will 则没有这个意思，如：

She is going to lend us her book.

He will be here in half an hour.

4.在有条件从句的主句中，一般不用 be going to, 而多用will, 如：

If any beasts comes at you, I'll stay with you and help you.