15. For register 7 bits
for memory 28 bits
remaining bits =40-(28+7)= 5
out of 5 bits one bit is used for mode and remaining 4 bits are used for opcode so 2^4 operations possible
so ans is b(16)
16.... out of 16 instructions n instructions are 2 address instruction
free opcode=16-n
one address instructions which uses memory operand is (16-n)*2^7
so option b