What form is required for an investor acquiring more than 5% of a reporting company's common stock?

For an investor acquiring more than 5% of a reporting company's common stock, the correct form to file is Form 13D. This form is intended to provide the SEC and the public with information about the beneficial ownership of the reporting company's shares, particularly when a party accumulates a significant stake. Filing Form 13D is a crucial disclosure requirement under the Securities Exchange Act of 1934, as it ensures transparency regarding significant ownership changes that can influence corporate control or management decisions.

The form requires detailed information, including the purpose of the acquisition, the source of the funds used for the purchase, and any plans or proposals for the company following the acquisition. This level of disclosure is vital for maintaining fair and informed markets, as other investors need to be aware of significant ownership stakes and potential changes in corporate governance.

The other forms listed serve different purposes: Form 10-K is an annual report that provides a comprehensive overview of a company’s financial performance; Form 8-K is used to report major events that shareholders should know about; and Form 4 is filed by insiders to report transactions in the company's stock but is not specifically required for acquiring more than 5%. Therefore, Form 13D is the correct form for reporting significant stake acquisitions.