National Association of Rocketry (NAR) Level 2 Practice Exam

Total impulse, not the thrust alone, determines the impulse class. Total impulse is thrust integrated over the burn time; in practice you multiply average thrust by burn time.

Here, 100 N of average thrust for 6 seconds gives a total impulse of 100 × 6 = 600 N·s. The standard impulse-class ranges place 600 N·s in the range labeled I (roughly 320–640 N·s). So this motor falls into impulse class I. The other classes correspond to lower or higher total impulses, so this one sits squarely in the I range.