Step 1: Understanding the Concept:
The question describes a method of irrigation where water is delivered to the field via a pipe network and then distributed to a specific portion of the field directly from an outlet on the pipe. This method avoids the need for open-air, earthen field channels, thereby reducing water loss from seepage and evaporation. We need to identify the name of the outlet device used for this purpose.
Step 2: Detailed Explanation:
Let's analyze the function of the components listed:
(A) Gate stands (or Gate Valves): These are valves used to control or shut off the flow within a pipeline. They are typically used in mainlines and sub-mains to manage the flow to different sections of the network, but they are not the final outlet for applying water to the field.
(B) Air vents (or Air relief valves): These are safety devices installed at high points in a pipeline. Their purpose is to release trapped air when the pipe is being filled and to allow air to enter when the pipe is being drained, preventing vacuum conditions and pipe collapse. They do not distribute water for irrigation.
(C) End plug: This is a cap used to seal the end of a pipeline. It stops the flow completely at the end of the line.
(D) Hydrants: A hydrant is an outlet device fitted onto a pipeline, equipped with a valve, that allows water to be discharged under pressure at specific points. In piped irrigation systems (like sprinkler, drip, or gated pipe systems), hydrants are the connection points where portable equipment (like sprinklers or gated pipes) is attached, or from which water is released to irrigate a specific area directly. They allow controlled access to the water in the pipe for irrigation without needing open channels. This perfectly matches the description in the question.
Step 3: Final Answer:
Hydrants are the outlets used to take water directly from a pipeline to irrigate a portion of a command area. Therefore, option (D) is the correct answer.