Final answer:
When nitrogen increases in volume due to a physical change, the density changes because density is mass per unit volume; the mass remains constant, but the volume increases.
Step-by-step explanation:
When an amount of nitrogen undergoes a physical change and increases its volume, it is evident that option D, the density changed, is what can be known about this physical change. A physical change involves a transformation in which the substance's physical properties are altered but its chemical identity remains the same. In this case, when nitrogen's volume increases, possibly due to heating, its mass remains the same while the space it occupies becomes larger. This leads to a change in density, because density is defined as mass per unit volume. There is no indication that the nitrogen's mass has increased, its color has changed, or its boiling point has increased as a direct result of this change.