Final answer:
A G3 is required to comply with the B149 Series of the Gas Code when working on gas-fired installations.
Step-by-step explanation:
When working on gas-fired installations, a G3 is required to comply with the B149 Series of the Gas Code. This code provides guidelines and safety standards for the installation and operation of gas appliances and equipment. It covers aspects such as venting, combustion air, fuel supply, and more.
The other options mentioned in the question, namely the Ontario Electrical Code and the Ontario Building Code, are not directly related to gas-fired installations. The Ontario Electrical Code provides regulations for electrical installations, while the Ontario Building Code specifies construction standards for buildings.
Therefore, the correct answer is A) B149 Series of the Gas Code, as gas-fired installations must comply with this code.