Answer:
The correct answer is: be installed; the free-rider problem.
Step-by-step explanation:
The free-rider problem is a conflict that takes place when, within a group of people, a small part of it is not willing to contribute equally or is not willing to contribute at all in something that could be beneficial evenly for all of them. In front of this situation, a consensus is held to determine how to enforce that small part to contribute accordingly.
In the case, as Mary and Jill are willing to pay more than what they should ($300 instead of only $250) it is likely that eventually the security system will be installed since two out of three votes are needed for that to happen.