Correct option is (a) Mn^{2+}
Explaination :::
number of unpaired electrons in Mn^{2+} =5
number of unpaired electrons in Fe^{2+} =4
number of unpaired electrons in Ti^{2+} =2
number of unpaired electrons in Cr^{2+} =4
Magnetic moment =\(\sqrt{n(n+2)} B.M.\) … Here "n" is the number of unpaired electrons .
The maximum number of unpaired electrons are in Mn^{2+} that is 5 therefore maximum magnetic moment is of Mn^{2+ }ion
Hence , (a) Mn^{2+} is correct