Black people
The apartheid bureaucracy devised complex (and often arbitrary) criteria in the Population Registration Act of 1945 to determine who belonged in which group. Minor officials administered tests to enforce the classifications. When it was unclear from a person's physical appearance whether the individual should be considered Coloured or Black, the "pencil test" was used. A pencil was inserted into a person's hair to determine if the hair was kinky enough to hold the pencil, rather than having it pass through, as it would with smoother hair. If so, the person was classified as Black. Such classifications sometimes divided families.

Q: What act determined what class a citizen belonged to?
the Population Registration Act of 1945