In basic biology, what term is used for an organism that is made up of a single cell?

Introduction / Context:
This question checks a core concept from cell biology and general science: how we describe organisms based on the number of cells they possess. Many microorganisms, including many bacteria and some algae and protozoa, are single celled, while larger plants and animals are multicellular.

Given Data / Assumptions:

• The question asks specifically for the term used for an organism with only one cell.
• Several related biological terms are given as options, including organism level terms and cellular components.
• You are expected to remember standard textbook definitions from school level biology.

Concept / Approach:
Organisms can be classified as unicellular or multicellular based on cell number. A unicellular organism consists of a single cell that carries out all the functions necessary for life, such as nutrition, respiration, excretion, and reproduction. Examples are many bacteria, amoeba, and certain algae. A multicellular organism has many cells that may be specialised for different functions, such as muscle cells, nerve cells, and leaf cells. Protoctist is a term used for organisms in the kingdom Protoctista, which can include both single celled and simple multicellular organisms. A chromosome is a structure inside cells that carries genetic information; it is not an organism.

Step-by-Step Solution:

Verification / Alternative check:
To verify, check any introductory biology textbook where the first chapters introduce cell theory. There you will find simple diagrams showing unicellular organisms like amoeba or paramecium, described explicitly as single celled, and labelled as unicellular. Diagrams of human beings, plants, and animals are labelled as multicellular. This confirms that the correct scientific term for a one celled organism is unicellular organism.

Why Other Options Are Wrong:

Multicellular organism is incorrect because it refers to organisms composed of many cells, such as humans, trees, and insects, which is the opposite of the condition in the question.

Protoctist is incorrect because it names a kingdom that includes various types of organisms and does not exclusively denote single celled life forms.

Chromosome is incorrect because it is a structure inside cells that contains DNA; it is not itself an organism.

Common Pitfalls:
A common mistake is to confuse the term unicellular with similar sounding words or to pick protoctist simply because it appears more technical. Students may also mistakenly think in terms of complexity, assuming any simple organism must be multicellular. Always focus on the definition given in the question: number of cells. When an organism consists of one cell, the correct term is unicellular; when it consists of many specialised cells, the correct term is multicellular.

Final Answer:
An organism that is made up of a single cell is called a Unicellular organism.