Both (a) and (b) shows geometrical isomerism
(a) shows geometrical isomerism and (b) shows optical isomerism
Both (b) and (c) shows geometrical isomerism
(b) shows geometrical isomerism and (c) shows optical isomerism
(a) [Ni(H2O)5NH3]2+ neither shows geometrical isomerism nor shows optical isomerism (b) [Ni(H2O)4(NH3)2]2+ shows cis-trans isomerism and (c) [Ni(H2O)3(NH3)3]2+ shows fac-mer isomerism which is nothing but geometrical isomers. Thus, (b) and (c) shows geometrical isomerism.