C0(carry at LSB) will always be 0 for addition , it will be 1 in case we want to do subtraction using an adder, so we need 15 FA for Most significant 15 bits & 1 half adder for two bits at least significant position.
we know that HA adds 2 bits & FA adds 3 bits, at LS position, there will be two bits only(we can ignore C0).