The correct answer is: A. in a unitary system, all powers are held by one government.
In a unitary system of government, the central government holds the majority of the power, and any administrative divisions (like regions or provinces) can only exercise powers that the central government chooses to delegate. In contrast, a federal system divides powers between a central government and various regional or provincial governments, with certain powers reserved for each level of government. The other options provided do not accurately describe the key distinctions between unitary and federal systems.