**Introduction:**
The Companies (Amendment) Act, 2000 introduced new provisions for the constitution of audit committees by public companies. One of the key criteria for the formation of an audit committee is the paid-up capital of the company. The correct answer to the question is option 'D', which states that the audit committee is required for a public company having a paid-up capital of Rs. 50 million or more.

**Explanation:**

**1. Companies (Amendment) Act, 2000:**
The Companies (Amendment) Act, 2000 was enacted to bring about changes and improvements in the functioning of companies in India. It introduced several provisions to enhance corporate governance and transparency in the operations of companies.

**2. Audit Committees:**
An audit committee is a committee of the board of directors responsible for overseeing the financial reporting process, internal controls, and audit functions of a company. The committee plays a crucial role in ensuring the accuracy and reliability of financial statements and compliance with applicable laws and regulations.

**3. Constitution of Audit Committees:**
The Companies (Amendment) Act, 2000 introduced a new section, namely section 292A, which mandates the constitution of audit committees by public companies. The purpose of this provision is to strengthen corporate governance and ensure proper financial oversight.

**4. Criteria for Audit Committees:**
Under section 292A, every public company is required to constitute an audit committee if it meets certain criteria. One of the key criteria is the paid-up capital of the company. The correct answer, option 'D', states that the audit committee is required for a public company having a paid-up capital of Rs. 50 million or more.

**5. Importance of Paid-up Capital Threshold:**
The paid-up capital threshold is an important determinant for the constitution of audit committees. It ensures that companies with a significant amount of capital have a dedicated committee to oversee their financial affairs. Companies with higher paid-up capital are likely to have larger operations and a greater impact on the economy, making it crucial to have proper financial oversight.

**Conclusion:**
The Companies (Amendment) Act, 2000 introduced provisions for the constitution of audit committees by public companies. The correct answer to the question is option 'D', which states that the audit committee is required for a public company having a paid-up capital of Rs. 50 million or more. This threshold ensures that companies with significant capital have proper financial oversight and enhances corporate governance and transparency in the operations of such companies.