Which of the following operations would be invalid for a string data type?

The operation of adding two strings together as numbers is invalid for a string data type because strings are designed to represent sequences of characters, not numerical values. In programming, operations involving arithmetic typically require numerical data types, such as integers or floats. When you attempt to treat strings as numbers during addition, the programming language does not inherently understand how to convert those character sequences into numerical values.

In contrast, operations like concatenating two strings, finding the length of a string, or converting a string to uppercase are all valid and commonly supported operations in programming languages. Concatenation combines strings into a single new string, finding the length returns the number of characters in a string, and converting a string to uppercase changes its character casing. These actions are explicitly supported for string data types, showcasing their versatility in handling character sequences.