Hola, why is "ser" used instead of "estar" in "Espero que haya tomado la decisión correcta y que sea más feliz con su nueva carrera."

Hola Terry

With the adjective feliz there is a tendency to use "ser" even when the sentence implies a change/result (this would generally trigger estar). You can also use estar here of course; we admit both answers as correct in the exercise, but for some reason this adjective is more commonly used with ser. 

Saludos

Inma