Which of the following statements about strings is true?

A string is defined as a sequence of characters, which can include letters, digits, symbols, and whitespace. This definition aligns with how strings are utilized in programming, particularly in C++. When you create a string variable, you are essentially creating a collection of characters that can be manipulated in various ways, such as concatenation, slicing, and searching. The main focus here is that strings are not confined to any particular type of character; they can hold a diverse mix of characters.

In contrast, the other statements do not accurately represent strings. A string is not limited to containing only numbers, as it can include any type of character. While some programming languages treat strings as immutable, C++ allows modifications to string contents; thus, calling a string immutable is misleading in this context. Lastly, while strings can represent numeric values when they contain digit characters, this representation is not the defining characteristic of strings themselves, which fundamentally relate to sequences of characters. Therefore, the statement that a string is a sequence of characters is the most accurate and foundational description.