Which action would most likely increase the greenhouse effect?

The correct answer is - releasing big amounts of CO2 into the atmosphere.

The CO2 is the gas that is by far the biggest contributor to the Greenhouse effect. This is due to the ability of the CO2 to trap the heat, thus bigger amount of heat remains into the atmosphere. So the bigger the amount of CO2 in the atmosphere, the bigger the heat, and vice versa, the smaller the amount of CO2, the colder it will be.

Be artificially introducing much bigger amounts of CO2 into the atmosphere, the result will be an increase in the Greenhouse effect. Unfortunately, this is already happening since the industrial revolution took place, and the Greenhouse effect has been steadily increasing.