At which animal biosafety level are no special barriers recommended other than a sink for handwashing?

The answer is A, as Animal Biosafety Level 1 (ABSL-1) is designed for use with well-characterized agents that are not known to cause disease in healthy humans or animals. At this level, work can be conducted in open laboratory settings without the need for special containment barriers. The primary safety requirement is having a sink for handwashing, which is a standard precaution to maintain hygiene. While personal protective equipment and general laboratory practices are still important, the risk of exposure to hazardous agents is minimal, allowing for a straightforward and less restrictive working environment. This makes ABSL-1 suitable for basic and educational research using non-pathogenic organisms.