One of the five groups A1C, C3E, E5G, G8I, I9K has been mis-set. Which group is it?

The opening letters A, C, E, G, I go on by two, the closing letters C, E, G, I, K likewise, and the numbers should be the odd numbers 1, 3, 5, 7, 9. The fourth group carries 8 where 7 belongs, so G8I is the faulty one — its letters are right and only its number is wrong. C3E and E5G carry the numbers the rule calls for; I9K carries 9, which is correct, and looks doubtful only if the numbers are read on from the faulty 8.