Explanation:

- Option 1: Size of the city – While a large city can make people physically farther apart, social distance is not defined only by size.

- Option 2: Distance between one’s residence and the place of work – This is about physical distance and commuting, not necessarily social distance.

- Option 3: Social heterogeneity – Social distance is mainly about the differences between groups in society, like race, class, or culture. The more diverse and heterogeneous a city is, the more likely people are to keep social distance from those different from themselves.

- Option 4: Inadequate public transport – While this causes inconvenience, it doesn’t affect social distance in the sociological sense.

Correct Answer: Option 3 – Social heterogeneity.