The correct answer is C) threshold limit values. Threshold limit values (TLVs) are the airborne concentrations of substances, such as hydrogen sulfide, under which nearly all workers may be repeatedly exposed without adverse health effects. TLVs are established by organizations like the American Conference of Governmental Industrial Hygienists (ACGIH) to provide guidelines for safe exposure levels in the workplace. The other options are incorrect because exposure limits and concentration limits are broader terms that do not specifically refer to the airborne concentrations considered safe for repeated worker exposure, and substance limit values is not a commonly used term in this context.