Step 1: Label each small square by position.
Call the four small squares in every box TL for top left, TR for top right, BL for bottom left and BR for bottom right, based on where the shaded square sits inside the 2 by 2 grid.
Step 2: Write the top line as a set of codes.
Top line, left to right: TL, TR, none, BL. So the shaded positions used in the top line are TL, TR and BL, plus one box with no shading at all.
Step 3: Write the bottom line as a set of codes.
Bottom line: A equals TR, B equals TL, C equals TR, D equals BR. So the bottom line uses the positions TR, TL, TR and BR.
Step 4: Find the position used in the bottom line but not in the top line.
TR appears in the top line at box 2 and TL appears in the top line at box 1, so options A, B and C are all accounted for, since C simply repeats the same TR code as A.
BR never shows up anywhere among the top line's four boxes. Only option D uses the BR position.
Final Answer:
Since BR is the one shaded position never used in the top line, pattern D is the missing one.
\[ \boxed{\text{Pattern D}} \]