The correct answer is option 'D' - Attorney General of India. The Attorney General of India is the highest-ranking law officer in the country and is appointed by the President of India. Let's understand why the Attorney General can be invited to give his opinion in Parliament.

Role of the Attorney General:
1. Legal Advisor to the Government: The Attorney General acts as the principal legal advisor to the Government of India. He provides legal advice and opinions on various matters referred to him by the President, Prime Minister, or any Ministry of the Central Government.

2. Representing the Government: The Attorney General represents the Central Government in important cases in the Supreme Court of India. He defends the government's position and presents arguments on its behalf. The Attorney General also represents the government in cases referred to the International Court of Justice.

3. Legislative Functions: The Attorney General has a crucial role to play in the legislative process. He is often called upon to provide legal opinions and interpretations of constitutional provisions, laws, and bills. His opinions help in shaping the legislative decisions.

4. Parliament's Right to Seek Advice: Under Article 76 of the Indian Constitution, the Attorney General can be invited by either House of Parliament to give his opinion on any legal matter. This provision recognizes the expertise and importance of the Attorney General's opinion in matters of legislation and constitutional interpretation.

5. Clarifying Legal Issues: The Attorney General's opinion aids the Members of Parliament in understanding the legal implications of proposed laws or policies. It ensures that the legislation is in line with constitutional provisions and does not violate any legal principles.

Importance of Attorney General's Opinion in Parliament:
1. Expertise and Experience: The Attorney General is a senior and experienced legal professional who possesses in-depth knowledge of constitutional law, administrative law, and various other legal aspects. His opinion carries immense weight due to his expertise in the field.

2. Constitutional Interpretation: The Attorney General's opinion helps in interpreting complex constitutional provisions and ensures that the legislation is in conformity with the Constitution. His advice helps in avoiding any legal complications or challenges to the validity of laws.

3. Ensuring Accountability: The Attorney General's presence in Parliament ensures transparency and accountability in the legislative process. His opinions provide legal guidance and prevent any unconstitutional or illegal actions.

4. Assisting Lawmakers: Members of Parliament may not always have a legal background, and therefore, the Attorney General's opinion helps them in making informed decisions. They can rely on his expertise to understand the legal implications of proposed legislations.

In conclusion, the Attorney General of India can be invited to give his opinion in Parliament due to his role as the principal legal advisor to the government, his expertise in constitutional law, and his ability to provide valuable insights into complex legal issues. His presence ensures that the legislation is legally sound and in line with the constitutional framework.