All propositions (B, C and D) should be eliminated, except for A. Britain is indeed a constitutional monarchy.
Proceeding by elimination:
B) is false because Britain does not have any single written constitution, it is governed by a collection of documents such as the 1689 Bill of Rights. We call this an "unwritten constitution."
C) is false because the U.S. Congress' power is limited by the President, who can exercise his right of veto.
D) is false because there is no inheritance right to the presidency in the U.S., one has to be elected to the position.
A) is indeed correct because a constitutional monarchy means that the monarch (king or queen) exercises power according to a written or unwritten constitution.
