The correct answer is option 3: a perfect black body is a good absorber and a good radiator.

To understand this, we need to know that a black body is an idealized theoretical object that absorbs all radiation incident upon it, without reflecting or transmitting any. It absorbs electromagnetic radiation across all wavelengths, from ultraviolet to infrared.

Option 1, stating that a black body is only a good absorber, is incorrect. A black body not only absorbs radiation but also emits it, making it an excellent radiator as well.

Option 2, stating that a black body is only a good radiator, is also incorrect. Although a black body is a good radiator, it must also be a good absorber to maintain equilibrium. It absorbs radiation and re-emits it.

Option 4, stating that a black body is neither a radiator nor an absorber, is incorrect. A perfect black body is both a good absorber and a good radiator.

In summary, a perfect black body is a good absorber of all incident radiation and a good radiator of its own radiation.