Hey, im having trouble with the below problem, i know which formulas must be used and understand how to calculate the ionic strength, which is equal to 0.25, but am stuck calculating the activity coefficient.

Problem : The permanganate ion activity in a

0.05M KMnO4 solution is

a)0.616

b)0.066

c)0.023

d)0.31

e)0.040

Any help would be great

thanks