Answer:
cambia
Step-by-step explanation:
You need to use the conjugation of the verb cambiar with the personal pronoun of ella (mi mamá)
Here is the translation to the complete sentence:
My mom changes the blue socks for the white socks.
Here is the indicative conjugation of the verb CAMBIAR in present:
Presente
yo cambio
tú cambias
él/ella/Ud. cambia
nosotros cambiamos
vosotros cambiáis
ellos/ellas/Uds. cambian
As you can see the options are conjugated with different personal pronouns cambio for yo, cambian for ellos and cambiamos for nosotros.