2 select line A and B will work as address lines, so we have 4 addresses (or words)

Each word will have 4 bits so memory required is 4 x 4 = 16 bits

In demultiplexer, inputs is inserted serially and then it gives multiple outputs which are in parallel form.

We need an adder for every bit. So  we should need m full adders. A full adder adds a carry bit to two inputs and produces an output and a carry.
                 But the most significant bits can use a half adder, which differs from the full adder as in that it has no carry input, so we need m-1 full adders in m bit parllel adder.