The correct anwer is D.Jill enjoys reading about American history, especially the last 40 years, or four decades, of the nineteenth century.
Step-by-step explanation
In history there are many words to refer a period, the basic one is a year that refers to 12 months or 365 days (or 366 in the case of a leap year). Also there are periods that grouped years, for example, the first is a lustrum, that is the same that five year; second is a decade, that is the same that ten years; the third is a quinquennium, that is the same that fifteen years; the fourth is a decanter, that is the same that fifty years; the fifth is a century, that is the same that one hundred years; the sixth is an age that is the same than various centuries (without unique quantity); the seventh is a millennium, that is the same that one thousand years; the eighth is an era, that is the same of various millenniums (without unique quantity) and the last that is Evo, that is a term used to no-limit period of time. According to the above, if Jill enjoys reading about American history, especially the last 40 years, that is a period that only can be related to decades because a quinquennium is ten years more than forty years. So, the correct answer is D. Jill enjoys reading about American history, especially the last 40 years, or four decades, of the nineteenth century.