Answer :
The building from the list provided with the highest likelihood of air pollution is a tightly sealed house with an unvented water heater. The school with leaks will let air exchange happen so that isn't it, and having a carbon monoxide detector only reports existing air pollution it doesn't prevent air pollution, it could have issues but we don't know. The tightly sealed house with the unvented water heater is a near sure thing.
when heater is on for long time without having ventilation then carbondioxide will be converted into carbon monoxide and causes air pollution.
So according to above explanation,
C. A tightly sealed house with an unvented water heater, is the correct answer.
