Concept:Political party systems are classified according to how many parties are present in a country.
Explanation:The party system refers to the arrangement of political parties within a state.
It is determined by the number of political parties that exist and contest elections.
Countries may practise a one-party, two-party, or multi-party system based on this number.
The electorate relationship, internal party structure, and mode of operation do not define the system type.
Answer:The correct option is D: the number of political parties in a country.