Margaery, who is constantly obsessing about her neighbor’s new things, is plagued with bad dreams. which of the four noble truths is she experiencing?

I believe the answer is: cause of suffering

In Buddhism's four noble truths, cause of suffering is caused by craving or lust which remain unfulfilled (usually worldly pleasures) which cause a person to be obsessed toward obtaining that lust. This could be seen in someone who is obsessing of other people's wealth, position,talent or lovers.