(a) The principle of natural justice

(b) The procedure established by law

(c) Fair application of law

(d) Equality before law     

Answer- a

Explanation:

The word ‘due’ in this clause is interpreted to mean ‘just’, ‘proper’ or ‘reasonable’, according to the judicial view. Thus, Due process of law, in essence is natural justice.

In the case of  Kartar Singh v. State of Punjab, the court held that the procedure contemplated by Article 21 is that the procedure must be “right, just and fair” and not arbitrary, fanciful or oppressive. In order for the procedure to be right, just and fair, it should conform to the principle of natural justice, that is, “fair play in action”.  

Hence, option a is correct.

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