The correct answer is C. Redundant code.
A redundant code is a code that uses more bits for each character than what is necessary. This is done to improve the reliability of the code by making it more resistant to errors. Redundant codes are often used in communication systems, where it is important to ensure that the data is received correctly.
Hollerith code is a code that was developed by Herman Hollerith in the late 19th century. It is a binary code that was used to represent data on punched cards. Hollerith code is no longer used in most applications, but it is still used in some legacy systems.
Gray code is a code that is used to represent data in a way that minimizes the number of changes that occur when the value of the data changes. Gray code is often used in digital circuits, where it is important to minimize the number of glitches that occur when the value of a signal changes.
Mnemonics are codes that are used to represent data in a way that is easy for humans to remember. Mnemonics are often used in programming languages, where they can help programmers to remember the names of variables and functions.
None of the above is not the correct answer.