Question

Ellos no __________ felices. (4 points)
Question 4 options:
1) estoy
2) están
3) estáis
4) estamos

Answer 1

2) están.
Estoy isn't right, because it refers to yourself and only yourself.
Estáis isn't right either because it basically means estás (you are), except in vosotros and a formal form.
Estamos is incorrect too because it refers to a group of people including you (basically, nosotros), and it means we are.
Hope this helps!