What did she warn me about while crossing the road?

The statement "To be careful while I crossed" accurately captures the intent of a warning regarding safety while crossing the road. Warnings are generally associated with precautionary advice, and in this context, being careful is crucial for ensuring one's safety among potentially dangerous traffic situations. This answer emphasizes the need for awareness and mindfulness in such scenarios, which aligns perfectly with the nature of the warning.

The other options suggest behaviors that would be unwise or reckless when crossing the street. Rushing, ignoring traffic, and acting without thought all imply a lack of caution, which contradicts the purpose of a warning. Therefore, the choice that focuses on carefulness stands out as not only sensible but also the most appropriate response to the query about the nature of the warning received.