Step 1: Observe the grid structure.
The figure is arranged in a $4 \times 4$ block pattern (four $2 \times 2$ groups). Each block contains four different shapes:
- Circle,
- Square,
- Triangle,
- Diamond.
Thus, every $2 \times 2$ block contains all four shapes without repetition.
Step 2: Shading rule.
Notice the alternation of filled and outline patterns:
- In the top-left block: Circle (filled), Square (outline), etc.
- In the other blocks: Shading alternates systematically so that within a $2 \times 2$ block, two shapes are filled and two are outlines.
Step 3: Identify missing block.
The missing bottom-right $2 \times 2$ block must:
1. Contain all four shapes (circle, square, triangle, diamond).
2. Follow the filling/outline alternation (two filled, two outlines).
Checking the answer options:
- (a) Contains Circle (outline), Triangle (filled), Square (filled), Diamond (outline) → matches both shape-completeness and shading balance.
- (b), (c), (d) either repeat shading wrongly or mismatch the placement of shapes.
Step 4: Confirm the match.
Thus, the correct block is option (a) because it completes the last $2 \times 2$ block with all four shapes in proper shading order.
\[
\boxed{\text{Option (a)}}
\]