b) 16
128 Registers: 7 bits is required to specify the register operand
256 M Memory : 28 bits is required to specify the memory operand
Total Length of the Instruction = 40 bits
So remaining 5 bits can be used to specify the opcode. So totally 32 different instructions are possible. Generally instructions with indirect addressing mode use specific notation to distinguish from direct addressing mode.
For eg : LD @A
Assuming this notation takes 1 bit. Something like 0 to specify direct addressing mode and 1 to specify indirect addressing mode. we are left 4 bits for opcode. Hence 24 = 16 instructions are totally possible.