Explanation:

- Option 1, Heterogeneity: A metropolis is often referred to as a racial and cultural melting pot due to its diversity. This diversity is a result of people from various racial, ethnic, and cultural backgrounds living together, leading to a blend of traditions, languages, and customs. This is what is meant by heterogeneity.

- Option 2, Mobility: While mobility is an important feature of metropolitan life, referring to the ease with which people move around or relocate for work or lifestyle, it does not directly explain the "melting pot" nature.

- Option 3, Density: This describes the number of people living in a certain area. Density indicates high population but doesn't explain cultural mixing.

- Option 4, Size: The physical or population size of a metropolis can be vast, but size alone doesn't address cultural diversity.