Correct option is (a) Mn2+
Explaination :::
number of unpaired electrons in Mn2+ =5
number of unpaired electrons in Fe2+ =4
number of unpaired electrons in Ti2+ =2
number of unpaired electrons in Cr2+ =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 Mn2+ that is 5 therefore maximum magnetic moment is of Mn2+ ion
Hence , (a) Mn2+ is correct