(In 2nd paragraph, corrects to show Starboard is one of ACI’s largest shareholders, not the largest shareholder) Dec 2 (Reuters) - ACI Worldwide Inc must explore all available strategic options to maximize value including a sale, activist investor Starboard Value said in a letter here to the payment systems company"s top executives published on Wednesday. Starboard Value, which is one of ACI’s largest shareholders with a stake of around 9%, urged the company to hire advisers and conduct a full and fair sale process. Its letter comes one day after ACI’s Analyst Day presentation. Starboard said it had expected ACI to project improvements in organic growth and profitability “so dramatic” that the choice to remain a public company would be obvious to shareholders. Instead, it said, ACI had presented a “conservative” guidance range and a gradual three-year timeline that implies it will take the company almost another two years to “get back to its 2019 revenue base and start producing new organic growth”. “If this new guidance is really the best that you can do over the next three years, then we believe that a sale of the Company is clearly a more attractive option for shareholders,” Starboard told ACI. ACI did not immediately respond to a request for comment. Its shares rose almost 10% in early Wall Street trading. Starboard said in October that it views ACI as an “attractive” takeover candidate, adding that the company has underperformed peers including Blackbaud Inc, Commvault Systems, and FireEye Inc. The investor said on Wednesday that ACI has been the subject of takeover speculation for many years, as the quality of its assets and importance to the broader payments ecosystem is obvious to most industry participants. It added that ACI’s management decided not to conduct a sale process last November, when it was rumored that ACI was approached by several different strategic and financial buyers. (Reporting by Noor Zainab Hussain in Bengaluru and Svea Herbst in Boston; Editing by Kirsten Donovan)