Answer:
A. Organ.
Explanation:
Cells make up
tissues, tissues make up an organ. Organs make organ system
According to Charles Darwin's theory of Evolution by natural selection, organisms that possess heritable traits that enable them to better adapt to their environment compared with other members of their species will be more likely to survive, reproduce, and pass more of their genes on to the next generation.
(n.d.). Retrieved from http://www.nas.edu/evolution/Definitions.html
One word answer:Adaptation:
Its Gene recombination during sexual reproduction
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