Which of the following is not an OS?

Pascal is not an operating system. Pascal is a high-level programming language developed by Niklaus Wirth in the late 1960s. The language is named after Blaise Pascal, a seventeenth-century French mathematician who constructed one of the first mechanical adding machines. Pascal is best known for its affinity to structured programming techniques. The nature of the language forces programmers to design programs methodically and carefully. For this reason, it is a popular teaching language.