The basic unit of classification / taxonomy is

Four statements are given below regarding the nomenclature of organisms. Which two statements are correct?
$(1)$ The first name denotes the species and the second denotes the genus.
$(2)$ $A$ species name used for one organism cannot be used for another organism.
$(3)$ Each word in a handwritten scientific name must be underlined.
$(4)$ It is mandatory to write the sub-species name after the species name for every organism.

Which of the following is incorrect regarding the rules of nomenclature?

Different taxa at the same level are exemplified by:

In the given taxonomic sequence,what could $A, B$,and $C$ be?

Write a short note on taxonomic categories.