The main advantage of a parliamentary form of government is that c) the executive remains responsible to the legislature. [Article 75(c)] Here's why the other options are not the main advantage: a) The executive and legislature actually work closely together in a parliamentary system, as the executive is drawn from the legislature.
b) Continuity of policy can be a benefit, but it's not the main advantage. Parliamentary systems can still be efficient in passing laws.
d) The head of government (Prime Minister) can be changed through a vote of no confidence in the legislature.