Sublimation is a phase transition where a substance changes directly from a solid to a gas without passing through a liquid phase. The question asks which substance exhibits this behaviour at standard temperature and pressure (STP). STP is typically defined as 0°C (273.15 K) and 1 atmosphere (101.325 kPa). Let's examine the options: A) Solid carbon dioxide (Dry ice): At atmospheric pressure (around 1 atm), solid CO₂ sublimes directly into gaseous CO₂ at -78.5°C. While this temperature is below the typical definition of standard temperature (0°C or 25°C), it readily sublimes at standard atmospheric pressure. Compared to other options, it's the classic example of sublimation under common conditions. Its triple point is -56.6°C at 5.18 atm; below the triple point pressure, it only exists as solid and gas. B) Water: At 1 atm, ice melts at 0°C and water boils at 100°C. Water exists as a liquid between 0°C and 100°C at 1 atm. Sublimation of ice can occur below 0°C, but water does not typically sublime from solid to gas at standard temperature (0°C or 25°C) and pressure (1 atm) as its primary transition. C) Liquid nitrogen: Nitrogen boils at -196°C at 1 atm. It is a gas at standard temperature. It doesn't sublime (solid to gas) at standard temperature and pressure; it transitions from liquid to gas at a much lower temperature. D) Iron: Iron is a solid at standard temperature and pressure. It melts at 1538°C and boils at 2862°C at 1 atm. It does not sublime under standard conditions. Solid carbon dioxide is the substance among the options that is well-known for undergoing sublimation at standard atmospheric pressure, even if the temperature is not strictly "standard room temperature". Given the context of such questions, dry ice is the intended answer for a substance showing sublimation behaviour at standard pressure.