The field of knowledge which will enable the bronze-age shopkeeper to keep track of his sales would be Mathematics. Option A is correct.
The Bronze Age refers to a period of time where major advancements occurred and bronze was used for tools. This age lasted from 3,300 BC(E) to 1,200-200 BC(E). In this age, cities were built, nations were set up, a new set of writing has been established during the bronze age.
