Choice "A" is correct. Revenues and gains are realized when assets are exchanged for cash or claims to cash. SFAC 5 para. 83.
Choice "c" is incorrect. Assigning depreciation in a production department is an example of allocating overhead. There is no realization associated with the assignment.
Choice "d" is incorrect. The realization concept is integral to accounting for revenues and expenses and is not connected to collection of receivables.
Choice "b" is incorrect. Assignment of overhead costs to products and thus to cost of goods sold is an example of matching. There is no realization associated with this assignment.