Which of these is not a string?

The correct answer identifies an option that is not a string. To clarify, a string is a sequence of characters enclosed within quotation marks. In this context, the quotation marks indicate that the content within them is treated as a string type.

The first option, "Hello," is clearly a string since it consists of multiple characters surrounded by double quotes. The third option, "3.0," is also a string because it includes numeric characters within quotes, both as a representation of a number and as text. The fourth option, '3', is a string as well, since it encapsulates the digit '3' in single quotes, maintaining the string format.

The choice indicated as correct, "A", consists of a single character in quotes. However, this does not change the fact that it still qualifies as a string. Therefore, any of the options with characters in quotes are indeed strings, including the selection highlighted. Thus, all the provided choices are strings in terms of data representation in programming contexts.