Concept:
Hydrants are outlets provided in a pipe irrigation system for delivering water to fields.
Step 1: In pipe irrigation, water is conveyed through pipes instead of open field channels.
Step 2: Water must be supplied to different parts of the command area as required.
Step 3: Hydrants are provided at suitable points to draw water directly from the pipeline.
Step 4: They help irrigate a particular portion of the command area at a time.
Step 5: Therefore, the correct answer is hydrants.
\[
\boxed{\text{Hydrants}}
\]