Hint: A ubiquitous small organism consisting of a single cell, which may or may not have a nucleus, cannot be seen with naked eye. Generally, it reproduces by asexual means and is part of nearly every habitat.
> Option A is incorrect. Hydra is a multicellular eukaryotic organism which belongs to the phylum Coelenterata. Chara is a charophyte-green algae genus within the Characeae family. They are multicellular and, owing to the stem-like and leaf-like structures, superficially resemble land plants.
> Option B is correct. Amoeba unicellular organism that has the ability to alter its form mainly by extending and retracting pseudopods and Paramecium is a genus of unicellular ciliates, generally studied as a representative of the ciliate group. They are both unicellular species, protozoa belonging to the Protista Family.
> Option C is incorrect. With a well-differentiated plant body, Marchantia and Riccia are both simple, primitive, multicellular, bryophytic plants. They are experiencing a generation-change with sporophytic and gametophytic stages. The gametophyte is the dominant stage of both genera in development. Their plant body is also a thallose, and is not distinguished between the stem, roots, and leaves.
> Option D is incorrect. Amoeba and Paramecium are the only unicellular organisms who belong to Kingdom Protista present in the above given options.
Hence, The correct answer is option (B).
The unicellular organism is a single-cell organism. This means all life processes occur in one cell, such as reproduction, feeding, digestion, and excretion. Amoebas, bacteria, and plankton are only several kinds of single cell species. Usually they are microscopic and can't be seen with the naked eye. They have simple structures with important life-related features. A few of them have locomotory organs such as flagella, cilia, and pseudopodia that aid in movement. Nutrition is primarily by phagocytosis (their cell membrane takes in food). And eukaryotic (present nuclei) unicellular species such as Chlorella (algae) have the ability to synthesise their own food using sunlight (photosynthesis). They reproduce by sexual as well as by asexual means. Some of them (e.g., bacteria) break into colonies and live.
Note: Cells are considered to be basic life components because they often come together to produce multicellular organisms, including plants or animals. While being much smaller, single cell organisms may perform some of the same complex behaviours as multicellular organisms.