The main advantage of the parliamentary form of government is that

1. A

   the executive and legislature work independently.

   (a) Executive and legislature work independently. That is the presidential / separation-of-powers claim, the opposite of parliamentary fusion. Not the key.

2. B

   it provides continuity of policy and is more efficient.

   (b) Continuity of policy and more efficiency. Presidential systems often claim continuity; parliamentary governments can fall mid-term. Not the distinctive advantage here.

3. C

   the executive remains responsible to the legislature.

   (c) The executive remains responsible to the legislature. Collective responsibility of the Council of Ministers to the Lok Sabha is the core advantage of the parliamentary form. This is the official key.

4. D

   the head of the government cannot be changed without election.

   (d) The head of government cannot be changed without election. A Prime Minister can be replaced without a general election if the House majority shifts. Not the key.

Summary. Official key is (c) the executive remains responsible to the legislature. In a parliamentary system the government is born of the lower house and can be removed by it (confidence, no-confidence, money-bill defeat). Independence of executive and legislature is presidential. Continuity without mid-term change is not guaranteed. Article 75(3) states collective responsibility to the Lok Sabha. That is the main advantage asked.