(D) Both (A) and (R) are true, but (R) is not the correct explanation of (A).
(A) is true — modern democracy cannot function without political parties. (R) is also true — without parties, elected representatives would be accountable only to their constituencies, not the country. However, (R) is a consequence of the absence of parties, not the explanation for why democracy needs parties.
The textbook states: "Elected representatives will be accountable to their constituency… But no one will be responsible for how the country will be run" — this describes what happens without parties, making (R) true. However, the core reason democracy needs parties is the broader necessity of gathering opinions, forming governments, and making policies — so (R) does not fully explain (A). Hence option (D) is correct.