The units place of 31 is 3.
The units place of 32 is 9.
The units place of 33 is 7.
The units place of 34 is 1.
The units place of 35 is 3.
The units place of 36 is 9.
The units place of 37 is 7.
The units place of 38 is 1.
The units place of 39 is 3.
Now we can clearly observe here that the units place of 3n belongs to the set 3,9,7,1.
The answer is option C (5).