Option (a) is correct: The concept of 'Due Process of Law' primarily means that the procedure through which a person is deprived of life or liberty must be fair, just, and reasonable, following the principles of natural justice. It ensures that laws are not only enacted properly but also applied fairly and reasonably.

Option (b) is not correct: The phrase 'Procedure established by law', used in Article 21 of the Indian Constitution, refers to any procedure laid down by law, even if it is unfair. However, after the Maneka Gandhi v. Union of India (1978) judgment, the Supreme Court read into Article 21 the American concept of ‘Due Process of Law’, ensuring fairness and reasonableness in both substance and procedure.

Option (c) is not entirely correct: While fair application of law is part of due process, the phrase ‘Due Process of Law’ has a broader meaning that encompasses substantive fairness, procedural justice, and conformity with natural justice. Hence, option (a) captures its essence more accurately.

Option (d) is not correct: Equality before law (Article 14) is a distinct constitutional principle and not equivalent to due process.