Basicity and nucleophilicity order will be same if donor atom is same.
Basically can be determined by knowing acidic strength.
Stronger the acid, weaker will be conjuate base.
These basicity order in given compound will be :
$OR > OH > RCOOH^-$
Nucleophilicity of anion is stronger than neutral molecule.
$(i)\;OH > H_2O, (ii) NH_2 > NH_3 (iii)OR > ROH $
Hence a is the correct answer.