Si Tomás _____ alegre hoy, es porque no tiene que tomar el examen.
Answer: está
Si Tomás está alegre hoy, es porque no tiene que tomar el examen.
If Tomás is happy today, it is because he does not have to take the test.
Explanation:
The basic difference between "ser feliz" and "estar feliz" is that "ser feliz" is a permanent state of mind and "estar feliz" refers only to a specific moment in which you talk. In this case, we talk about today. Today Tomás is happy, but maybe he is not always happy.
[tex]\textit{\textbf{Spymore}}[/tex]