Some relatively simple eukaryotes, such as the adder's tongue fern, may have many more chromosomes than a more complex eukaryote, such as a mammal. What might this suggest about the size and organization of chromosomes in different species?

Low gene density in the ferns.

Explanation:

The amount of DNA present in the organisms varies from species to species but is not necessary that the amount of DNA in organisms is associated with the complexity of the organisms.

It is usually expected that the higher the amount of DNA, the higher the complexity or lower DNA content lower the complexity but is not always true as in the given case in which the simple organism have more chromosomes.

This could be due to the low gene density or the number of genes per million base pairs in the Ferns that is few DNA codes for the trait.

Thus, Low gene density in the ferns is the correct answer.