Explanation
The conjunction 'but' should be used to connect the two sentences. The first sentence states a positive quality about the girl while the second sentence states a negative quality. Hence, they should be joined by a connector which brings out antagonism or presents a contrast.
'Because' is used to state a reason for something which can't be the case here. Hence, because can be ruled out.
'Not' can't be used to connect these two sentences. It is used to compare two similar contexts, say she is proud, not humble. Pretty and proud are two very different qualities; hence, 'not' can't be used to connect the two given sentences.
'While' is used to talk about two things happening simultaneously. When used to present a contrast, it means 'whereas'. This meaning doesn't fit in the context of the two statements. Hence, it can also be ruled out.