As referred to in the text, legacy systems are:

Legacy systems are typically defined as traditional mainframe-based business information systems that have been in use for a significant period of time. These systems often serve critical business functions and may involve a considerable investment in terms of time and money, making it challenging for organizations to replace or upgrade them.

Mainframe systems have been integral in managing complex data processing tasks and handling large volumes of transactions, which is why they are often categorized as legacy systems. They are characterized by their stability and reliability, but their underlying technology may be outdated, limiting the integration with modern applications or the ability to leverage newer technologies.

While the other options pertain to various types of computing systems, they do not accurately embody the full definition or characteristics of legacy systems. Electronic spreadsheets, for instance, are versatile tools but do not represent the comprehensive nature or longevity of legacy systems. Similarly, the mention of pre-1990 Wintel systems and those found on older ASPs does not encompass the broader definition of legacy systems as traditional mainframes embody.