Step 1: Analyzing the figure.
The given black solid figure resembles a partial square, and we need to identify which option, when added, completes the square shape. The correct option will fit seamlessly to form a perfect square.
Step 2: Analyzing the options.
(A) Option A: This does not fit the given figure correctly to form a square.
(B) Option B: This does not complete the figure into a square.
(C) Option C: This is correct, as it perfectly fits the given figure to make a square.
(D) Option D: This does not form a square when added to the figure.
Step 3: Conclusion.
The correct answer is (C) because it correctly completes the given figure into a square.