A hereditary system of government is a system where the position of power is passed down from one family member to another based on bloodline or lineage. In this type of system, the ruler or leader is usually determined by birthright, rather than by merit or election. This can be seen in a monarchy, where the title of king or queen is passed down from one generation to the next within the same family. It can also be seen in aristocracy, where political power is held by a small group of wealthy and privileged families. Therefore, the correct option is "a monarchy".