Concept:Citizens' political participation refers to the lawful activities through which people influence government decisions and public affairs.
Explanation:A political party is the chief organisation that legally mobilises citizens to take part in politics.
It provides a clear platform for voting, campaigning, contesting elections, and joining the government.
Through political parties, citizens can express their views, seek representation, and shape national policies.
A vigilante group and a cooperative society are not formed primarily for political participation.
A militant pressure group only influences government from outside, but it does not offer a full opportunity to contest elections or hold power.
Therefore, the best practice of citizens' political participation is found under a political party.
Answer:D. political party