Pensar vs Creer

Hola Shafagh

Your sentence using "pienso" is also correct and that is included as a correct answer in the exercise. Kwizbot may have shown you on its feedback screen some model answers to this. It may have not included all of them in the system because there are lots of different options to use for "pretty" (guapa, bonita, linda, hermosa...), but you should have seen one of them using "pienso que...", so you can mark yourself right on that one.

In terms of creer and pensar, when they mean I think that... creer is a bit more commonly used but it depends on the person. For me personally, "pienso que..." sounds just a little bit more formal, but it is a personal choice as I said.

Saludos

Inma