The correct answer is: Former republics are more difficut to rule than hereditary states, and they will rebel if they are not tighly controlled.
This is the only sentence that is completely related to the passage, it summarises the things mentioned in it.
First it states "Former republics are more difficult to rule than hereditary states", Machiavelli describes how in an hereditary state under the rule of a prince, the people is used to being told what to do, to see things as permanent, they are not accustomed to changes. Imagine that before the prince ruled, a great number of his ancestors did, for the people it has become something natural. So they are in some way afraid of freedom, it is easy for them to follow instructions without questioning anything.
In the other hand in a republic, things change as people is used to have a more direct participation in the decisions taken. They are not used to just accept the rules but to help create them. "...they will rebel if they are not tighly controlled." The author's suggestion is to destroy them as they won't accept the rule in an easy way, they will question everything and conspire against domination.
Here we can see that the strategies for the control of each type of state is different.