Masks in Project 1

In project 1, I don’t understand what question 3 (masks) is asking us to do.

You need to design a parity filter (PF).

You get x[n - 1:0] as the input. You need to sum up the digits of x, which is equivalent to counting the number of $1$s in x.
If the sum is divisible by 2, your output y[n - 1:0] should be the same as x[n - 1:0].
If the sum is not divisible by 2, y[n - 1:0] should be all zeros.

In part 1 of the question, you need to design a general \mathrm{PF}(n) and analyse the design for cost and delay.
In part 2, you need to design \mathrm{PF}(4), i.e. your input and output will be 4 bits each. You need to implement this design in the pf4 circuit.