What is considered the velocity of one knot?

The velocity of one knot is defined as one nautical mile per hour. Nautical miles are specifically used in maritime and air navigation because they are based on the Earth's geometry and align with degrees of latitude and longitude, making them more relevant for navigation purposes compared to standard miles and kilometers. This makes option B the correct answer.

The other choices do not apply in the context of nautical navigation. One mile per hour, while a valid measure of speed, is not the correct definition for knots. One kilometer per hour provides a metric perspective but, again, does not correspond to the nautical measurements. One light year per hour is not a frequently used unit of speed in practical applications, particularly in aviation or marine navigation, and is vastly larger than any of the other standard speed measures. Thus, the definition of a knot as one nautical mile per hour stands as the correct choice in this context.