Meaning: 'Complex' refers to something that is made up of many interconnected parts, indicating that biological systems are not straightforward but rather intricate.

Wrong options:

• system: The word 'system' is a noun that refers to a set of connected things, but it does not describe the nature of the biological systems as intricately as 'complex' does.
• national: The term 'national' pertains to a nation and does not relate to the intricacy of biological systems, making it contextually irrelevant here.
• extra: The word 'extra' implies something additional or beyond what is usual, which does not accurately describe the nature of biological systems in this context.

Correct answer: organisation

Explanation: The term 'organisation' is appropriate as it refers to the structured arrangement of biological systems, which is essential in the context of natural selection. It fits grammatically as a noun that complements the subject of the sentence.

Meaning: 'Organisation' refers to the way in which parts are arranged or structured, indicating that biological systems have a specific arrangement that aids in their function.

Wrong options:

• presence: The word 'presence' indicates existence but does not convey the structured arrangement of biological systems, making it less suitable in this context.
• registration: The term 'registration' refers to the act of recording or enrolling, which does not apply to the context of biological systems and their arrangement.
• structures: While 'structures' could relate to biological systems, it does not capture the idea of arrangement or systematic organization as effectively as 'organisation' does.

Correct answer: functions

Explanation: The word 'functions' fits grammatically as it describes the roles or purposes that biological systems serve, which is crucial for survival and reproduction. It aligns well with the context of natural selection.

Meaning: 'Functions' refer to the specific roles or activities that biological systems perform, indicating their usefulness in survival and reproduction.

Wrong options:

• cultures: The term 'cultures' relates to social behaviors and practices, which is not relevant to the biological context of survival and reproduction.
• samples: The word 'samples' refers to portions or examples of something, which does not convey the idea of roles or purposes in biological systems.
• introductions: The term 'introductions' implies bringing something into a context, which does not fit the context of biological systems and their roles in survival.