Option A is incorrect: Proportion of literates in the population While education is important, it's an individual skill, not a measure of social interaction or trust.
Option B is incorrect: Stock of buildings This falls under physical capital, which refers to tangible assets like buildings and infrastructure.
Option C is incorrect: Size of population in the working age group This is a demographic factor and doesn't tell us anything about social connections or trust.
Option D is correct: Social capital refers to the intangible assets of a community or society. It's not about physical resources or demographics, but rather the quality of relationships and connections between people. Option D, the level of mutual trust and harmony, directly reflects this concept.