Answer: To answer the question I see in the screenshot provided, it would be "Variation of traits from sexual reproduction.
To answer the question of what an adaptation is in my own words:
An adaptation is a trait that a species evolves to obtain over a long period of time, due to the competition of resources with other species. This can also be explained with "Survival of the fittest".
Explanation:
What evidence must be considered when determining whether or not a trait is an adaptation:
1) A trait that is an adaptation is always heritable, as in it goes from every parent to every child.
2)An inheritable trait can skip generations, an adaptation cannot.
3) Adaptations are evolved traits that provide a specific function that enables an organism to survive. A longer bird beak allows a bird to obtain more food when easily obtained food is scarce.
4) An adaptation trait must enhance the fitness of an organism. (Fitness refers to whether an organism can produce viable offspring that has better advantages due to the trait)
5) The appearance of the trait and how the species uses the trait must occur at the same time. This shows that the trait was evolved for the purpose for which it's used. A bird with a long beak doesn't need a long beak if food is abundant.