Which of the following is a domain of eukaryotic organisms?

What was the major contribution of the scientist $R$.$H$. Whittaker in $1969$?

Classification of organisms on the basis of fossil records that plays an important role in the elucidation of evolutionary relationships is:

Which type of classification system is Whittaker's five-kingdom classification?

Statement $S$: The classification system proposed by Woese is called the three-domain system. Reason $R$: Woese divided organisms into three domains: Archaea,Bacteria,and Eukarya.

Why are the classification systems changing every now and then?