For computers based on three-address instruction formats, each address field can be used to specify which of the following: (S1) A memory operand (S2) A processor register (S3) An implied accumulator register

In a three-address instruction format, an instruction explicitly specifies three operands (typically two source operands and one destination operand):

Each address field can specify:

1. A memory operand (address in memory), or

2. A processor register.

An implied accumulator register is used in zero-address or one-address instruction formats (where the accumulator is implicitly understood by the opcode and not specified in an explicit address field).

Therefore, each address field can specify either S1 (a memory operand) or S2 (a processor register).

Hence, (A) is the correct option.