Which of the following is considered a unit of classification?

Hierarchical classification means

Which of the following taxonomic categories contains the least number of similarities?

In the given diagram,what do the parts labeled $P$ and $Q$ represent respectively?

What is required for the formation of a Genus?

How many of the following statements are correct?
$(A)$ Each taxonomic category is a unit of classification.
$(B)$ Taxonomic groups are distinct biological entities.
$(C)$ Genera are aggregates of closely related families.
$(D)$ Both vegetative and reproductive characters are used to characterise the families.