A.I. Explanation:

Correct answer: basis

Explanation: The word 'basis' serves as a noun that indicates the foundation or underlying principle of Darwin's theory. It fits grammatically as it is the subject complement following the verb 'was,' and contextually, it aligns with the idea of 'survival of the fittest' being a fundamental concept in organic evolution.

Meaning: 'Basis' refers to the underlying support or foundation for an idea or theory.

Wrong options:

• debate: 'Debate' is a noun that refers to a discussion or argument, which does not fit as it does not indicate a foundational concept in this context.
• functions: 'Functions' is a plural noun that refers to the specific activities or roles of something, which does not align with the idea of a foundational principle.
• physically: 'Physically' is an adverb that describes something in a physical manner, which does not fit grammatically or contextually as it does not relate to the concept of a basis.
• differences: 'Differences' is a plural noun that refers to distinctions between things, which does not fit as it does not indicate a foundational concept.

Correct answer: differences

Explanation: The term 'differences' is a plural noun that refers to the variations among living things resulting from evolution. It fits grammatically as it serves as the subject of the clause, and contextually, it aligns with the idea that evolution leads to diversity among species.

Meaning: 'Differences' refers to the distinctions or variations between organisms that arise through evolutionary processes.

Wrong options:

• debate: 'Debate' is a noun that refers to a discussion or argument, which does not fit as it does not describe the outcome of evolution.
• functions: 'Functions' is a plural noun that refers to the roles or activities of organisms, which does not align with the concept of variations resulting from evolution.
• physically: 'Physically' is an adverb that describes something in a physical manner, which does not fit grammatically or contextually as it does not relate to the concept of differences.
• basis: 'Basis' is a singular noun that refers to the foundation of a theory, which does not fit as it does not describe the outcome of evolution.

Correct answer: primarily

Explanation: The word 'primarily' is an adverb that indicates the main source or focus of evidence supporting the theory of evolution. It fits grammatically as it modifies the verb 'has come,' and contextually, it emphasizes that the fossil record is the main source of evidence.

Meaning: 'Primarily' means mainly or chiefly, indicating the primary source of evidence.

Wrong options:

• debate: 'Debate' is a noun that refers to a discussion or argument, which does not fit as it does not indicate the manner in which evidence has come.
• functions: 'Functions' is a plural noun that refers to the roles or activities of organisms, which does not align with the context of evidence gathering.
• physically: 'Physically' is an adverb that describes something in a physical manner, which does not fit grammatically or contextually as it does not relate to the source of evidence.
• basis: 'Basis' is a noun that refers to the foundation of a theory, which does not fit as it does not describe the manner in which evidence has come.
• differences: 'Differences' is a plural noun that refers to variations, which does not fit as it does not describe the source of evidence.