Arrange these species in their decreasing order of nucleophilicity.
(a) C > D > B > A (b) B > A > C > D
(c) A > B > C > D (d) C > A > B > D

Nucleophilicity depends on the tendency of a group or atom give electrons for making  a chemical bond with electrophilic centre. It decreases  along the period (reason is electronegativity) and increases  down the group because of the increase in size and decrease in electronegativity.
Oxygen is the most electronegative atom in period 2  of the periodic table .
Carbon is least electronegative atom and N is more in between C and O .
So most nucleophilic is CH3 - and then NH2 - .
CH3O - has  electron releasing group and hence more rich in electron density and better nucleophile than OH- .
to order must be:(a)