Alex stayed at home.

The correct answer is: He

In the above sentence we will use 'he' because he/she, I/me, they/them, he/him, she/her, we/us, are personal pronouns. Personal pronouns are pronouns that refer to specific individuals and groups. Whenever we want to use a pronouns other than human beings then we use 'it' in that case, for human beings we use 'he/she'.Here, we can understand that Alex is a boy's name and everytime a boy or a man is referred, we will use 'he'.