The correct answer is A) ventilate the space, then check the battery, spark plugs, carburetor, and fuel line. When troubleshooting a gasoline engine's failure to start, the first step is to ensure proper ventilation to avoid the risk of fire or explosion from fuel vapors. Then, you should systematically check the key components that are essential for the engine to start, including the battery, spark plugs, carburetor, and fuel line. This methodical approach helps identify the root cause of the problem. The other options are incorrect because: B) priming with ether can be dangerous, C) checking spark by disconnecting plug wires is not the first step, and D) breaking the fuel line to let gas run in the bilges is unsafe and could create a fire hazard.