Which is the most effective when fighting a Class A fire deeply embedded in a pile of dunnage?

Correct Answer: Option A — Water

Water is the most effective agent for deep-seated Class A fires because of its high latent heat of vaporization. It penetrates porous materials like dunnage to cool the fuel below its ignition temperature, whereas gaseous agents like CO2 or Halon only provide surface cooling and lack the necessary penetration.