There are numerous roots in this tap root system. Kindly suggest or share a research paper where I can clarify the doubts as to which root should be chosen for sectioning and for further root anatomical studies?
But it will depend on your research question won't it? What are you trying to discover about the root system?
If you want to see how cells are divided at the junctions where 3 or 4 roots diverge from the main root, section it there. If you want to see how root tips develop into mature roots, section from the tip back to the branch point.
If you have doubts about where to look, perhaps you'll have to just go for it and look at the entire length of a root from tip to origin. A lot of work, but then you'll know.
It really depends on what you want to know about the roots. Are you interested in primary root anatomy or secondary? Are you interested in adventitious roots? We can answer your question better if we know your hypothesis.