Question

1.She prefers not to eat sweet food.

Answer 1

第一个是对的.prefer常见用法有：

prefer sth to sth

prefer doing sth.to doing sth

prefer to do sth.rather than do sth

这里的prefer to do 中的to do是不定式,不定式的否定是not to do ；就像我们学的tell sb not to do sth,ask sb not to do sth