Hola Marcos

"Pienso que no" is correct.

"No pienso" doesn't have this meaning/use here in this context. The only way you can use "no pienso" is with a whole sentence, not as a set answer. So you need to continue: No pienso que hoy vaya a llover; No pienso nunca en ella; No pienso volver a llamarla., for example.

"No lo pienso" doesn't work in this context either, but you can use "No lo creo" though meaning the same as we explain in the lesson. However this answer would be more colloquial. You can actually drop the "lo" too.

- ¿Crees que Cristina vendrá a la fiesta? 

- No creo / No lo creo. (I don't think so, I doubt it)

Saludos