Correct choice is (b) 0.5 m
For explanation I would say: Electric field due to a charge q at a distance r is \(\frac {q}{kr^2}\)
and electric potential at that point is \(\frac {q}{kr}\). Therefore, \(\frac {q}{kr^2}\)=32 and \(\frac {q}{kr}\)=16. Dividing these two equations, we get r=0.5m. If the medium is air, k=1. Thus we can get the value of q=0.89 C.