What describes a ruler who has absolute power over a nation?

A ruler who has absolute power over a nation is best described as an autocrat. This term is derived from the Greek "autos," meaning "self," and "kratos," meaning "power" or "rule." An autocrat exercises complete control over governance, often without any formal checks or balances. This authority allows the autocrat to make decisions unilaterally and enforce laws without the need for consent from a governing body or the populace.

The context of the other options highlights their unrelatedness to the concept of governance. "Avid" generally refers to a keen interest or enthusiasm, not a governing style. "Auxiliary" refers to something that provides supplementary support which does not pertain to authority or power structure. "Astringent" typically describes substances that contract bodily tissues or denotes something that has a sharp or bitter quality, unrelated to political power. Thus, the precision of the term "autocrat" in describing absolute power makes it the correct choice.