What condition is critical for the safe use of penta?

The correct choice highlights the importance of using a closed system when mixing penta, which is a wood preservative. This condition is critical for safety as it minimizes the potential for exposure to harmful vapors or chemical spills that can occur during the mixing process. A closed system prevents the release of toxins into the air, protecting both the user and the environment.

Using a closed system also helps in maintaining the integrity of the chemical composition of penta. Any interaction with air or moisture could potentially alter its effectiveness or create hazardous byproducts. Therefore, proper mixing in a controlled environment is essential for both safety and efficacy when working with wood preservatives like penta.

The other options do not provide the necessary safety measures or operational procedures required for the correct handling of penta. For instance, applying under high temperatures or direct sunlight may not be safe or effective, and mixing freely with other chemicals poses significant risks of unwanted reactions.