Step 1: Identifying the correct option:
In sociology, "race" is classically defined as a category of people distinguished by shared, inherited physical characteristics - such as skin colour, facial features and hair type - that a society treats as socially significant.
Step 2: Why the other options are wrong:
Birth is the basis of ascribed status in general (including caste), not race specifically. Wealth is the basis of class, not race. Achievements define achieved status, which is the opposite of the ascribed, physically-based nature of race.
Final Answer:
Physical characteristics (Option B)