Which one of the following countries is widely regarded as a classic example of a federal form of government?

Introduction / Context:
Federalism is a system of government in which power is divided between a central authority and various constituent units such as states or provinces. Constitutional law and political science often refer to certain countries as classic or textbook examples of federal systems. This question asks the learner to identify which country among the given options is widely recognised as such a classic example.

Given Data / Assumptions:
- The focus is on the federal form of government.
- The options list China, United States of America, Cuba, Belgium, and Sri Lanka.
- Only one of these is traditionally presented in textbooks as the classic model of federalism.

Concept / Approach:
In comparative politics, the United States of America is often treated as the archetypal or classic federal state. Its Constitution clearly divides powers between the federal government and the states, with a written Constitution and strong judicial review. China and Cuba follow socialist single party systems with unitary structures, while Sri Lanka is also largely unitary though it has experimented with devolution. Belgium is a federal state today, but in many introductory texts the United States is still cited as the standard model when explaining federalism.

Step-by-Step Solution:
Step 1: Recall the meaning of a federal government as one where power is constitutionally divided between a central authority and constituent units. Step 2: Identify that the Constitution of the United States of America is one of the oldest written constitutions and clearly embodies a federal system with states having their own powers and the federal government having enumerated powers. Step 3: Understand that China is a socialist republic with a unitary structure and strong central control by the Communist Party, not a classic federation. Step 4: Recognise that Cuba is also a socialist state with a unitary system of government. Step 5: Belgium is indeed a federal state, but in basic Indian polity exams, the United States is usually cited as the primary classic example of federalism. Step 6: Hence, the best answer that matches the phrase classic example is the United States of America.

Verification / Alternative check:
Standard Indian polity textbooks like those used for civil services preparation explicitly describe the United States as a classic federal system, in contrast to India's quasi federal structure. These sources often start their explanation of federalism by referring to the United States model, which confirms this answer.

Why Other Options Are Wrong:
China: Operates as a highly centralised socialist republic with a unitary system; provincial governments do not enjoy constitutional autonomy comparable to states in a federation.
Cuba: Also a socialist unitary state with centralised authority, not a classic federal system.
Belgium: It is a federal state in contemporary times, but the question emphasises the widely known classic example, which is the United States of America in most standard references.
Sri Lanka: Has a unitary constitution with some devolution of power to provinces, but it is not recognised as a federal state.

Common Pitfalls:
Learners sometimes choose Belgium because they know it is a federal state, but they overlook the wording classic example that usually points to the United States in exam oriented polity discussions. Another mistake is to equate regional autonomy or devolution with full federalism and therefore mislabel countries like Sri Lanka as federal. A clear understanding of constitutional division of powers helps avoid such errors.

Final Answer:
The country widely regarded as a classic example of a federal form of government is the United States of America.