The correct answer is A) To prevent the government from being controlled by one branch.
The developers of the US Constitution were trying to prevent one individual from having too much power. This was based on the Americans experience as British colonists under the rule of King George III. King George III was able to pass laws for the American colonists without their consent in the parliament.
This is why America has a federal government that contains a legislative, executive, and judicial branch. Each of these branches has the ability to check the power of another branch. For example, if a president vetoes a bill, the legislative branch (Congress) can override this veto with a 2/3rds vote in both the House of Representatives and the Senate.