What primarily causes heat transfer in conduction?

The primary mechanism of heat transfer in conduction is through molecular vibration contact. In this process, when molecules of a substance are heated, they begin to vibrate more rapidly. These vibrations are transmitted through collisions with adjacent molecules, allowing heat to flow from the warmer areas to the cooler areas of a material. This direct transfer of thermal energy occurs without any movement of the material as a whole, distinguishing conduction from other forms of heat transfer like convection or radiation.

In conduction, heat is effectively transmitted through solid materials. This process is crucial in understanding how heat spreads through structures during a fire, which can significantly influence fire behavior and safety strategies. For instance, during a fire event, understanding how heat conducts through walls or floors can help investigators determine fire spread patterns.

The other options involve different mechanisms of heat transfer. Flame movement pertains to convection, where hot gases rise and create a current. Radiant energy reflection involves the transfer of heat via electromagnetic waves, which is a separate phenomenon from conduction. Gas expansion relates to the movement of gases, typically in the context of convection, not conduction. Therefore, the focus on molecular vibration contact as the means for heat transfer in conduction solidifies its identification as the correct answer.