Step 1: Set up coordinates on the square.
Label the square PQRS with P at bottom left and R at top right, and divide each side into 6 equal parts using the step size of the notches in Panel I.
This gives a 6x6 grid of unit cells, 36 cells in all.
Step 2: Read off the shown piece.
Going row by row from the top edge (S to R) to the bottom edge (P to Q), mark which of the 36 cells in Panel I are black.
Counting gives 25 black cells, so the missing piece must fill the other 11 cells, always in the same fixed arrangement.
Step 3: Plot the missing 11 cells.
Row 2 has empty cells at columns 2, 5 and 6, row 3 has empty cells at columns 2, 3, 4 and 5, row 4 has an empty cell only at column 4, and row 5 has empty cells at columns 2, 3 and 4.
Together these 11 cells form one connected outline, 4 rows tall and 5 columns wide.
Step 4: Match this outline against Panel II.
Overlay the same 4x5 grid on each option and compare cell by cell with the outline from Step 3.
Options (i), (ii) and (iv) each differ from the outline in at least one cell, but option (iii) lines up with it in every cell.
Final Answer:
The shape in option (iii) is the exact complement of Panel I, so it is the other part of the square.
\[ \boxed{\text{Answer} = \text{(iii)}} \]
