The odds of a:b (a to b) represents that for each number 'a' of people that exceeds the maximum data, there is a number 'b' or people that don't exceed it. So the total number of people will be 'a + b'.
Converting this odd to probability, we need to divide the number of people 'a' by the total number of people 'a + b', therefore the denominator of the fraction is 'a + b'.
So the correct option is C.