Assertion (A): Higher the category greater is difficulty of determining relationship to other taxa at the same level.
Reason (R): From species to kingdom, the number of common characteristics goes on decreasing.

Assertion (A) is correct because, in biological taxonomy, as you move to higher taxonomic categories, such as from species to family, order, class, and so on, the taxa at those higher levels encompass a broader range of organisms with more diverse characteristics. Therefore, determining relationships and finding common characteristics that unite taxa at the same higher level can become more challenging.

Reason (R) is also accurate. As we move up the hierarchy of biological classification from species to kingdom, the number of common characteristics typically decreases. This is because at the species level, organisms within the same species share a significant number of common characteristics, as they are closely related and can interbreed. However, as you move to higher taxonomic levels, such as genus, family, order, and so on, the common characteristics become more general and shared by a wider range of taxa. At the kingdom level, organisms within the same kingdom (e.g., Animalia or Plantae) share very few common characteristics, as the kingdom encompasses a wide diversity of life forms.

Both (A) and (R) are true and (R) is the correct explanation of (A).