(D) Through Elections and Voter Support
Political parties ensure accountability to the public through elections and voter support, as people can replace parties that fail to perform by voting them out of power.
The passage states that in a democracy, "people can replace them (party leaders)" through elections. Elections are the primary mechanism by which parties are held accountable — if they underperform, voters withdraw support. Options A and B are not mentioned as accountability tools; Option C (partisanship) actually weakens democracy. Option D is the only constitutionally grounded answer.