Which of the following quantities represents mass?

Mass is a fundamental quantity that measures the amount of matter in an object, and it is usually expressed in units such as grams (g), kilograms (kg), or similar units in the metric system. In this case, 0.34 g is clearly a measure of mass because it represents a quantity in grams, which is a direct unit of mass.

On the other hand, the other options are measurements of force or weight rather than mass. The value given in pounds (12.0 lbs) is a measure of weight, which can depend on gravitational force. Similarly, the values of 1600 kN (kilonewtons) and 899 MN (meganewtons) are measures of force, not mass. This is because newtons are defined as the force required to accelerate a mass of one kilogram at a rate of one meter per second squared. Thus, the option that correctly identifies a unit of mass is 0.34 g.