The correct answer is D) an electrical circuit in which a spark is incapable of causing the ignition of a given explosive mixture. Intrinsically safe equipment is designed to be incapable of releasing sufficient electrical or thermal energy to cause ignition of a hazardous atmospheric mixture under normal or specified fault conditions. This means that the electrical circuit cannot produce a spark or other ignition source that could ignite flammable gases or vapors in the surrounding environment. Option D accurately describes this defining characteristic of intrinsically safe equipment. The other options are incorrect because they do not fully capture the concept of intrinsic safety. Limiting electrical circuits to low current (option B) or low voltage (option C) can help reduce ignition risk, but do not necessarily make the equipment intrinsically safe. Option A refers to instrumentation in hazardous areas, but does not specify that the equipment must be incapable of causing ignition.