Question

All eukaryotic unicellular organisms belong to
(A) Monera
(B) Protista
(C) Fungi
(D) Bacteria

Answer 1

Hint: The five-kingdom classification is done based on certain basic variations among animals and one such important factor is the presence of a true nucleus i.e. the organism is prokaryotic or eukaryotic. Based on the structure of the nucleus, the unicellular organisms are put into two different kingdoms.

Complete answer:
All unicellular organisms having eukaryotic nuclear structures belong to the kingdom Protista. They are not plants, animals, or fungus but some may closely related to either of them. Originally this Kingdom contains prokaryotic organisms as well but later on, they were separately grouped under the kingdom monera.

Additional Information: The Monera kingdom consists of organisms that have a single prokaryotic cell that governs all its activities as an organism. A major example of such an organism is Bacteria, which are single-celled and have no true nucleus. The division of monerans from protists was first proposed by Ernst Haeckel.
Fungi is another kingdom of eukaryotic organisms and they are characterized by chitin in their cell walls. They were not included in Plantae or Animalia due to the reasons that they are heterotrophs like animals yet they possess a cell wall in their cells that resemble plants. Therefore, they were separately grouped under kingdom fungi. Some important examples of such organisms are yeast, molds, etc.
Bacteria are generally classified under kingdom monera as they are single-celled prokaryotic organisms. They are really small measuring around a few micrometer lengths. But though they are microscopic they possess great variation and can survive in the harshest of environments.
So, the correct answer is "(B) Protista".

Note: Before the introduction of the kingdom Protista, there were only two kingdoms namely plants and animals. But protists didn't resemble any one of them completely. Therefore, John Hogg proposed the classification of a separate kingdom in 1850 named Protoctista. Later on, in 1866 the proposal of a third kingdom named Protista happened and it was done by Ernst Haeckel. But originally he included monerans in that group which later got separated.