Solution:(i) "Modern democracy is a party democracy (katz)". Richard S. Katz, a political scientist has written extensively on the role of political parties in modern democracies.
(ii) "Democracy is unthinkable save in terms of parties". This statement was given by E.Е. Schattschneider in his work Party Government (1942).
(iii) "Parties are the core institutions of democratic politics" (Lipset) Seymour Martin Lipset, a sociologist and political scientist highlighted the importance of political parties as care institutions in democratic politics.
(iv) The existence of political parties is often as a necessary condition for the existence of modern democracy. This statement is not by Robert Dahl.