Right answer:
b. marcar una casilla
If you're using an e-mail on a browser, you might be signed in automatically if your browser saved your username or password. You can do this on your browser. Regarding the sentence marcar una casilla the word marcar is an infinitive verb, the most basic form of verbs in Spanish. Finally:
Cuando abres tu correo-e, puedes marcar una casilla para que la computadora recuerde tu contraseña.
