The correct answer is 3) Consuls had to agree on all decisions, which prevented any one person from gaining too much power.
The consuls in Rome were chosen by the Senate and elected by the people - comitia centuriata - both of them had equal power and the other could undo what the other had ordered, this was a power called Imperium.
They served for one year and could not be elected again for the next ten years. All of this prevented people from gaining too much power.