The correct answer is C) increases.
A catalytic combuster always increases the efficiency of a fireplace.
A catalytic combustor uses a ceramic honeycomb design of oval shape. It is coated with some noble metal to resist the heat. The smoke can be burned with this application. A catalytic combustor can increase the efficiency by 10% and the air pollution is reduced up to 75%, That is why a catalytic combuster always increases the efficiency of a fireplace.
The other options of the question were a) decreases and b) has no effect.