Mark the black square PQRS as a 6 by 6 grid of small cells, using the notches in Panel I as guide lines. The piece shown in Panel I already covers 25 of the 36 cells, so the missing piece must be made of exactly the remaining 11 cells, in a fixed connected pattern.
Trace that leftover pattern cell by cell. It forms a shape 4 units tall and 5 units wide, with two arms at the top joining a solid middle band and a foot at the bottom left. Now compare this outline with each option in Panel II.
(i): the bottom row of this shape has one extra filled cell compared to the leftover pattern, so it does not match.
(ii): the left cell of the bottom row is missing here where the leftover pattern needs it filled, so this does not match either.
(iii): checking all four rows against the leftover pattern, every filled and empty cell lines up exactly.
(iv): the third row has an extra filled cell and the bottom row is missing a cell on the left, so this fails too.
Only shape (iii) reproduces the leftover 11 cell pattern exactly, so option (C) is the other part of the square.
