Sticking To The Passage

In origin the agrarian myth was not a popular but a literary idea, a preoccupation of the upper classes, of those who enjoyed a classical education, read pastoral poetry, experimented with breeding stock, and owned plantations or country estates.

The origin is literary

The people who spent a bunch of time thinking about this is the upper classes

The passage suggests that the agrarian myth originated:

A in literature.

B on country estates in Europe.

C on small farms owned and worked by yeoman farmers.

D among the urban elite who romanticized the virtues of the simple life of the farmer

The key here is realizing that it wants the origin. The passage tells us it is literary. The preoccupation (or what someone spends a lot of time thinking about) is irrelevant since that doesn’t match the question well.