If natural selection over the time period of eukaryote evolution has favored a dna architecture in eukaryotes that keeps genes turned off except when needed, this structure would be

If natural selection over the time period of Eukaryote evolution has favored a DNA architecture in Eukaryotes that keeps genes turned off except when needed, this structure would be nucleosomes.

Explanation:

The nucleosome is the fundamental unit of "packaging" of eukaryotic DNA. This structure contains chromatin, which is a fundamental substance of the cell nucleus and consists of the combination of DNA with proteins. In eukaryotic cells the structure of chromatin is the organization in which DNA is presented in the cell nucleus, and is responsible for masking (protecting) certain sequences, while allowing free access to identical ones. Local chromatin has effects on gene expression, forming special structures that actively bring amplifiers closer to promoters. The active or open chromatin may contain regulatory sequences, promoters, transcribed sequences and regions bound to insulating chromatin proteins. Heterochromatin is a chromatin that is usually located in the vicinity of the nuclear envelope, here genes that are not normally expressed are located, although there are portions of DNA in the constitutive heterochromatin that are transcribed.

answer: a. a gene that causes a specific trait.