This triangle is called a 45-45-90 triangle, and the side ratio for the sides opposite to the 45-45-90 degree angles respectively are 1-1-sqrt2
Now we know one side is 6 and its opposing the 45 degrees angle and BC is opposing the 90 degrees angle the ratio between the sides are 1:sqrt2
Therefore BC=6*sqrt2=6sqrt2
Thus B is the correct answer!
Hope this helped any questions just leave them in the comments of this answer.