What identifies a stock or mutual fund using a series of letters?

A ticker symbol is a unique series of letters that represents a particular stock or mutual fund on the stock market. This shorthand allows investors and traders to easily identify and trade stocks within a global exchange. The ticker symbol is typically one to five letters long and is crucial for efficient buying and selling of stocks, as it simplifies communication and transactions on trading platforms.

The use of a ticker symbol helps investors to quickly find and reference specific investments without needing to provide lengthy names, which may be complex and less memorable. For example, Apple Inc. is represented by the ticker symbol "AAPL," making it easy for anyone to recognize and trade.

In contrast, other terms such as Investment Code, Stock Identifier, or Market Tag do not have standardized meanings in the context of stock trading and often do not refer to the unique alphanumeric representation of stocks or mutual funds. Hence, the ticker symbol stands out as the correct answer in this context.