All the given words are adjectives.
'Regular' means 'recurring at uniform intervals'. 'Orderly' means 'neatly and methodically arranged'. 'Methodical' means 'done according to a systematic or established procedure'. 'Steady means 'regular, even, and continuous in development, frequency, or intensity'. 'Customary' means 'traditional', 'regular' or 'usual'. It also means 'according to the customs or usual practices associated with a particular society, place, or set of circumstances'.
All the above words describe something which happens in an orderly fashion or according to certain pre-determined rules or system. They are used to describe a noun which is predictable. As these words have similar meanings, options A, B and D are incorrect.
'Vague' is an adjective which is used to describe something which is not clearly expressed, known, described, or decided. It describes a noun which is unpredictable. As this word conveys a meaning which is opposite to that of the other given words, option C is correct.