Final answer:
Density is the physical property of aluminum at STP that remains constant across different samples, as it is an intensive property and not dependent on the amount of substance. Option A.
Step-by-step explanation:
At STP (Standard Temperature and Pressure), the physical property of aluminum that always remains the same from sample to sample is density. Density (ρ) is an intensive property of matter which means it does not depend on the amount of the substance present. The density of aluminum is 2.70 g/cm³ at 20°C, and this value will not change whether the sample is large or small. Density is calculated by dividing the mass of an object by its volume (ρ = m/V). Therefore, options such as mass, length, and volume, which are extensive properties, will vary depending on the sample size, unlike density.