Your ISP $($Internet Service Providers$)$ gives you an IP address, and it's public knowledge which ISPs have access to which IP addresses, thus:

O a$.$ geotargeting cannot be used to serve advertising.

O b$.$ IP addresses are primarily used for identifying device manufacturers.

O c$.$ geotargeting is illegal due to privacy concerns.

O d$.$ IP addresses can be used in geotargeting.

Oe$.$ GPS is based on IP addresses