There are a couple of ways to do this.
Take the first digit and multiply by 2 but express the result in octal (base 8), so 2×6=14, then add using octal arithmetic 68+14=104, because 8+4=14 octal; then 6+1+carried 1=10 octal.
Another method is to divide 68 by 8=8 which is 10 octal remainder 4: 104, answer A.