Which fixed expression below is standard in modern British English (mostly often used)?
(a) on an average
(b) on the average
(c) on average
Looking through some old English Grammar books, including "A concise English Grammar for Foreign Students" by C.E.Eckersley I encountered the expression on an average. (2-edition, 1958, p.14)
In both Longman Dictionary of Contemprorary English and Cambridge Dictionaries Online only on average is given.
So, is (a) incorrect or archaic?
What about (b)?