What is the ruling on women exposing the palms of their hands, or their hands and forearms, to non-mahram men in the marketplaces?

A woman exposing her hands and forearms in marketplaces is a reprehensible act and a cause of fitna (temptation/strife), especially if they are adorned. Allah the Almighty says: (And let them not strike their feet so as to make known what they conceal of their adornment).

Believing women should fear Allah and adhere to Allah's command to the wives of the Prophet (peace and blessings be upon him): (And abide in your houses and do not display yourselves as [was] the display of the former times of ignorance. And establish prayer and give zakah and obey Allah and His Messenger).

Men, as guardians, should fulfill their trust by guiding women and preventing them from causes of fitna. They are responsible for this before Allah.