Yes you can use it on geocities However, be careful the host is not running any competitive ads, which would violate the adsense terms if you were to also run adsense on the page.
hmmm.... yes. geocities pages have yahoo ads there a you can see... so if you place adsense, you wil violate the TOS... and i dont think google will approove geocities pages.
they approved my first site which was a geocities site back in the day. you really can't get enough traffic to make any money at a geocitie site.