Step 1: Understanding the Concept:
One of the primary functions of a Stock Exchange is "Safety of Transactions." This ensures that investors are not cheated.
Step 2: Detailed Explanation:
The Assertion (A) is true because stock exchanges provide a regulated platform for trading. The Reason (R) explains \textit{why} it is safe: because the legal framework and exchange rules strictly regulate members and their dealings. Since (R) provides the underlying cause for (A), it is the correct explanation.
Step 3: Final Answer:
The correct alternative is (a).