What might indicate that driving in the left lane is illegal?

Answer: B — Road signs prohibiting it

Driving in the left lane can be illegal if there are road signs specifically prohibiting it. These signs are designed to inform drivers of the rules of the road in that area, ensuring that traffic flows smoothly and safely. For example, in many jurisdictions, there are signs that indicate "Keep Right," instructing drivers to use the right lane for general travel while reserving left lanes for passing or specific types of vehicles. The other factors, while they might influence driving behavior, do not necessarily indicate legality. A lack of other vehicles does not mean it's illegal, as traffic conditions can change rapidly. Reduced speed limits might impact which lane you should be in, but they don't inherently restrict lane usage. The presence of an emergency vehicle typically calls for yielding rather than outright prohibiting lane usage. Thus, clear road signs serve as the most decisive indication regarding the legality of driving in the left lane.