Step 1: Understanding the Given Solid.
The given shape is part of a cube, and we are tasked with finding which of the options will complete the cube when joined.
Step 2: Analyzing the Options.
Upon inspecting the options, it is clear that option (a) A will perfectly fit with the existing part of the cube, completing the full solid.
Step 3: Conclusion.
Thus, the correct answer is option (a) A, as it will complete the cube.
\[
\boxed{\text{The correct answer is (a) A.}}
\]