this is just an attempt takin 0dbspl i.e., the threshold of hearing at the 1.4th km or 1400m... wil get 4.96m .. i.e., approx option (e)5...this can be done using the inverse square law...
L2=L1-20logD2/D1 where L2=49dbspl, L1=0dbspl, D2=1400 and D1 is got as 4.96.. i hope this is rite