Final answer:
The correct order of decreasing priority for the given groups is CO₂H > CH₂OH > CH₂CH₃ > CH₃ option b, following the CIP priority rules based on the atomic number of atoms directly attached to the chiral center or double bond.
Step-by-step explanation:
The correct order of the groups in terms of decreasing priority is CO₂H > CH₂OH > CH₂CH₃ > CH₃. This order is determined using the Cahn-Ingold-Prelog (CIP) priority rules, which consider the atomic number of atoms directly attached to the chiral center or double bond.
In the case of these groups, the carboxylic acid group (CO₂H) has the highest priority because it contains the atom with the highest atomic number, oxygen. This is followed by the alcohol group (CH₂OH), the ethyl group (CH₂CH₃), and finally the methyl group (CH₃) which has the lowest priority.
When applying the CIP rules, remember that a multiple bonded atom is equivalent to the same number of single bonded atoms when it comes to priority assignment. For example, CO in the carboxylic acid group counts as two oxygen atoms.