The correct answer is C) 5.5 seconds. The minimum period of roll that does not exceed the design limits of the legs is determined by the natural period of roll, which is a function of the vessel's geometry and stability characteristics. For a single amplitude rolling of 3 degrees, the natural period of roll should be at least 5.5 seconds to avoid exceeding the design limits of the legs. The other options are incorrect because: A) 1.5 seconds is too short and would likely exceed the design limits. B) 3.0 seconds is also too short and would likely exceed the design limits. D) 7.0 seconds is longer than necessary and may be overly conservative.