Political parties are primarily defined in which of the following ways?
a. a candidate
b. a state
c. a law
d. an issue

Well, in general, world-wide I would say that the best answer is D.  - political parties unite people who have the same views and would like to promote these views exercising political powers.

On a local scale, the parties need to follow laws (and in some countries, the law might only allow one party) and they might even only exist to support a certain candidate (but usually it's the other way: the candidate represents the party).

Its a whole new world