Explanation

(a)
All those groups which contain at least one pair of non-bonding electrons on the atom adjacent to the benzene ring, are $ortho\ and\ para$ directing. Among the given options, all are $ortho\ and\ para$ directing but their capacity of $ortho - para$ direction follows the order
[image:13_image250.emf]
Hence, [image:13_image251.emf]is the strongest $ortho - para$ directing groups.