Decentralization refers to systematic delegation of decision-making authority from top management to lower levels. It is not mandatory; it is a policy decision made by the organization depending on its structure, culture, and business needs. This policy decision is made at the discretion of top-level management, who decide how much authority to delegate and to whom. Hence, both statements are correct, and the reason explains the assertion.
textbf{Final Answer:} (A) Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of Assertion (A).