Is the following code legal?

In programming, the legality of code is often determined by whether it adheres to the syntax rules of the language being used. Option C correctly points out that for the given code, if a variable named s is intended to be a string, and if the value of i is set to 3, then accessing s.substring(i, i + 1) can indeed lead to an out-of-bounds error under certain conditions.

When calling substring(i, i + 1), it is crucial that the starting index i and the ending index i + 1 both fall within the valid range of indices for the string s. If s is shorter than 4 characters, the index 3 would not be a valid starting point, since indices start at 0. Thus, this operation would result in an error, confirming the correctness of this option.

This understanding of string operations and index bounds is essential in programming, as it highlights the importance of ensuring that your indices correspond to actual characters within the string to avoid runtime exceptions.