It's letter C. When ever you read "breaks down" always think decomposition. When you read that compound get together to form "a single" compound, think synthesis.
The correct answer that would best complete the given statement above would be option C. Here is the complete statement. In a decomposition reaction, the compound breaks down; in a synthesis, <span> reaction, a single compound is assembled from two or more substances. Hope this answer helps. </span>
