Which of the following statements is correct?

Select one:

a$.$ Set B of all strings of equal number of a$\text{'}$s and b$\text{'}$s defines a regular language

b$.$ L $($A$*$ B$)$N B gives the set A

c$.$ A$=[$If a b$\backslash$deg $|$n $=0,1,2,3.)$ is regular language

d$.$ None of these