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

a$.$

geotargeting cannot be used to serve advertising.

b$.$

geotargeting is illegal due to privacy concerns.

c$.$

IP addresses are primarily used for identifying device manufacturers.

d$.$

IP addresses can be used in geotargeting.

e$.$

GPS is based on IP addresses