A castrated cockerel is called a "capon". A cockerel is a young male chicken, and when it is castrated, its testicles are removed, which affects its growth and development. Capons are typically larger and fattier than uncastrated chickens, and their meat is considered to be more tender and flavorful. The terms "boiler", "pullet", and "hen" refer to other types of chickens, but they do not describe a castrated cockerel specifically. Therefore, the correct answer is "capon" when referring to a castrated cockerel.