In terms of rocket motors, specific impulse is defined as:

Specific impulse is an essential measure in rocketry that evaluates the efficiency of a rocket motor. It is defined as the total impulse delivered per unit weight of the propellant consumed during that time. This measurement reflects how effectively the propellant is converted into thrust, with higher specific impulse indicating more efficient engines that can produce more thrust for the same amount of propellant.

Selecting the option that states that specific impulse is the total impulse divided by a unit weight of propellant accurately captures this concept. Total impulse represents the overall thrust generated over the duration of the motor's operation. By dividing this impulse by the weight of the propellant, specific impulse provides a direct measure of how much thrust is produced for each pound (or kilogram) of propellant used. This metric allows engineers and aerospace professionals to compare different rocket engines and propellants based on their performance and efficiency.

While concepts like total impulse and thrust per unit time are related to rocket performance, they do not precisely define specific impulse as it pertains to propellant usage and efficiency. Thus, the answer chosen correctly encapsulates the definition and significance of specific impulse in rocketry.