Since the slips are picked up one after the other without replacement, they have distinct numbers on them 4 distinct numbers can be arranged among themselves in 4! ways. (i.e. 24 ways). Of these 24 ways only one arrangement is in ascending order.
∴ The required probability = 1/24.
Using Venn diagram
People who are O +ve = people who have only Rh = 44)
ABCF, ABF, ABEF, ABDCF, ABDEF, ADCF, ADCEF, ADEF, ADEF.