Which of the following corporate actions is mandatory for the investor?

A buyback, or share repurchase, occurs when a company buys back its own shares from the marketplace. While it can benefit shareholders by potentially increasing the value of remaining shares, participation is not mandatory for investors, as they can choose to retain their shares without any obligation to sell back to the company.

A rights offer allows existing shareholders to purchase additional shares at a discount before the company offers them to new investors. While it provides a chance for investors to increase their stake, participation in a rights offer is entirely optional and not mandatory, as shareholders can choose not to exercise their rights.

A purchase offer is an invitation from an acquiring company to purchase shares from shareholders, often at a premium. Similar to a buyback, acceptance of a purchase offer is voluntary for investors, and they are not required to sell their shares if they choose not to participate.