we can see it as two movements: one on the north-south axis and one on the west-east axis.
On the north-south Axis, the hiker went 8 km north and 4 km south, so in total the hiker went 4 km north (we can subtract one for the other and the direction remains that which was bigger)
On the west-easth Axis, the hiker went 6 km east and 6 km west, so in total the hiker didn't move on this axis! or, he/she moved but then "came back"
So in total, the hiker only moved4 km north - answer B.