Ser, I guess.
In spanish "estar" is like doing something (I'm workin'/Estoy trabajando) or feeling something (I'm sad/Estoy triste).
"Ser" is like being something or someone (I'm a doctor/Soy un doctor).
That's why jokes like this bellow can't happen:
_Hello dad, I'm working on my essay.
_Hello "I'm working on my essay", I'm dad.
(I know it's a stupid example, I'm sorry).
SORRY THE BAD ENGLISH, I'M STILL LEARNING IT.
