For n=1 we have 3 in unit place
n=2 we have 9 at unit place
n=3 we have 7at unitplace
n=4 we have 1 at unitplace
n=2 we have 3 at unit place similarly it can be seen that only 3,9,7,1 appear at unit place
In other words there are 4 possible outcomes and probabilityof 3 appearing is is 1/4.