Under what condition can extra bacteria sample bottles be carried in the truck's cab?

Carrying extra bacteria sample bottles in a truck's cab is permissible when they are in a clean enclosed container. This condition ensures that the samples are protected from contamination and damage, which is crucial for maintaining the integrity of the samples until they can be processed in a laboratory. Clean and enclosed containers minimize the risk of exposure to environmental factors, spills, and other contaminants that could compromise the samples and yield unreliable test results.

The other options do not adequately address the need for maintaining the cleanliness and safety of the samples during transport. Marking bottles properly, while important for identification, does not replace the necessity of proper containment. Allowing drivers to carry bottles at their discretion could lead to unsafe handling practices, and stating that they are not allowed at all overlooks the proper protocols that permit their transport under specific, safe conditions.