Arizona State University (ASU) CSE110 Principles of Programming Exam 1 Practice

The statement "Scanner sc = new Scanner(System.in);" is valid because it creates a new instance of the Scanner class that is linked to the standard input stream, which is generally the keyboard. This allows the Scanner to read user input from the console. In Java, the Scanner class requires a specific input source to function, and using "System.in" as the argument provides the Scanner with access to the standard input stream, making it capable of reading input data.

The other choices are invalid for various reasons. For instance, creating a Scanner with no arguments (as in the first choice) is not permitted, as the Scanner needs an input source. The third choice attempts to pass a String directly into the Scanner constructor in a way that is not syntactically correct; the correct usage would require creating a Scanner that reads from a String using the right constructor that takes a String as input. The last choice is also invalid as it tries to create a Scanner instance without the `new` keyword and does not include a valid input source.

Thus, "Scanner sc = new Scanner(System.in);" stands out as the only correct and valid way to instantiate a Scanner for reading from standard input in Java.