What is the correct way to concatenate a string variable (UserName) with " is online"?

Concatenating strings involves combining them into a single string. The correct option, which involves taking the variable UserName and appending the string " is online" to it, underscores a fundamental aspect of string manipulation in most programming languages.

Using the variable directly, as in the first option, allows the program to dynamically include whatever value has been assigned to UserName. This means if UserName were assigned a value such as "Alice", the resulting string would be "Alice is online".

In contrast, other options do not achieve the intended result. For instance, adding the string "is online" before the username places the phrase incorrectly, resulting in "is onlineAlice", which doesn't convey the intended message. Not quoting "is online" in other options leads to an error because it is treated as an undeclared variable and would generate a compilation or runtime error. Finally, quoting UserName in an option suggests that the literal string "UserName" is used rather than the value of the variable, resulting in "UserName is online" instead of an actual user's name. Thus, the given correct answer accurately reflects how string concatenation should function with a variable and a string literal.