The correct answer is B) 1 - 3 tenths. An un-reinforced vessel can safely navigate in ice provided the concentration does not exceed 1 to 3 tenths. This is because un-reinforced vessels are not designed to withstand the pressures and forces exerted by heavy ice concentrations. Operating in ice concentrations greater than 3 tenths can pose significant risks of damage or even sinking for un-reinforced vessels. The other options are incorrect because: A) 9-10 tenths is too high for an un-reinforced vessel, C) 4-6 tenths is still too high, and D) 6-8 tenths is also too high for safe navigation of an un-reinforced vessel in ice.