Pipes of different diameters and lengths connected end to end to form a pipe line are referred to as compound pipes. Let's explore the reasoning behind this:

1. Understanding the Term:
   • Compound pipes involve different segments of pipes with varying diameters and lengths.
   • These segments are assembled in series to make a continuous pipeline.
2. Why Other Options are Incorrect:
   • Equivalent Pipes: This term typically refers to a single pipe that has the same hydraulic behavior as a combination of pipes. It does not denote physically joining different pipes end to end.
   • Branched Pipes: This involves a main pipe with branches or offshoots, which doesn't fit the description of pipes connected end to end.
   • Pipes in Parallel: This configuration refers to pipes laid side by side such that fluid can divide and flow through multiple paths simultaneously, which contrasts with the series arrangement described.
3. Conclusion: Based on the definitions and characteristics above, it is clear that the correct answer is compound pipes. These pipes, when connected end to end, form a continuous flow path despite their differing specifications.

Thus, the term "compound pipes" correctly describes a pipeline formed by connecting pipes of various diameters and lengths end to end.