选C
Her son is to study English as his major the moment he has finished high school.
高中毕业的时候,他决定今后选择英语作为自己的专业.
is to 表示将要
从时间长短来说
be about to < be going to < is to
be about to 可能是一个小时内发生的事情
be going to是一两天内的事情
is to 没有大概的限制,更合题意.
希望我的回答能对你有帮助.
谢谢o(∩_∩)o