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The group of organisms, which neither come undereukaryotes nor prokaryotes are known asprotists. They comprise plants and animals that are devoid of tissues. They can be multicellular or unicellular. Based on modern taxonomy, they are regarded as aparaphyletic group comprising protophyta, protozoa, and slime molds.

The genus, which represents the cellular slime molds is dictyostelium, and the organisms coming under this genus can be both multicellular and unicellular.

Ulva is a multicellular green alga.

One of the largemultinucleate slime molds is the plasmodium.

Therefore,ulva, dictyostelium and plasmodium do not oppose the statement that the size of the cell is confined to unicellularity. Thus, optionsa, b and c are incorrect.

A marine green alga is caulerpa. It is a single-celledalga. The size of the cell is very large, that is, about several meters in size. This opposes the statement that thesize of the cell is confined by unicellularity.

Thus, the correct answer is option d.