Which specific problem led Rome to change from a monarchy to a republic?

The king did not hold enough power.
The king was unable to win wars.
The king mistreated the citizens.
The Senate mistreated the citizens.

The specific problem that led Rome to change from a monarchy to a republic was that the king mistreated the citizens.

The specific problem that led Rome to change from a monarchy to a republic was the mistreatment of its citizens by the monarchy. The last three options listed in your question all involve mistreatment of the citizens, but it should be noted that the mistreatment was primarily attributed to the king rather than the Senate. It was the widespread dissatisfaction with the monarchy's rule and mistreatment of the people that eventually prompted the Roman citizens to demand a change in their system of government, leading to the establishment of the Roman Republic.