Which type of pipe nipple is threaded over its entire length?

The correct answer is that a close nipple is threaded over its entire length. A close nipple is generally used to connect two fittings directly without any space between them, making it ideal when a compact connection is needed. Its full threading allows for a secure and leak-proof connection, which is crucial in various plumbing and piping applications.

Other types of nipples, such as reduced nipples and spigot nipples, have specific applications that do not require threading along their entire length. For example, a reduced nipple has a different diameter on either end, while a spigot nipple is designed for specific types of fittings that do not necessitate full threading. Therefore, the unique characteristics of the close nipple make it the appropriate choice for being fully threaded throughout.