The key difference between parliamentary and presidential democracies is:
D. There is less separation of executive and legislative power in a parliamentary democracy.
In a parliamentary system, the executive branch is derived from the legislative branch, and the two are intertwined. The head of government (the prime minister) is typically a member of the legislature and is accountable to it. In contrast, in a presidential system, there is a clearer separation of powers, with the executive (the president) being elected independently from the legislature.