The farmer is working . . . . . . . . his field.

A. at

B. in

C. on

D. over

Answer: Option B

Solution(By Examveda Team)

The correct preposition to be used here is 'in'.
In cases like this, the 'field' is a place or location, so the preposition is 'in'.
For example - He works in a bakery.
When the object in the sentence is an object (a thing), then the preposition would be 'on'.
For example - He works on computers.