Final answer:
The "killer app" introduced in 1983 that made the IBM PC a must-have business tool was Lotus 1-2-3.
Step-by-step explanation:
The application program introduced in 1983 that became the "killer app" making the IBM PC an essential tool for business was Lotus 1-2-3. This spreadsheet application followed the earlier program VisiCalc and clearly demonstrated the utility of personal computers in the business environment. Lotus 1-2-3's powerful capabilities in data analysis, management, and graphical representation of data made it a must-have for businesses, thus ensuring the success of the IBM PC in the competitive computer market of the 1980s.