De facto segregation is when people are separated in fact but not by:

A. Law

B. Choice

C. Borders

D. Tradition

"De facto" means "in fact, factually" - that something is, indeed, taking place. It is customarily used to denote a situation which is not dictated by law ("de jure") but which happens anyway - so the best answer is A. Law - this segregation is not required by law, but it takes place anyway, for example for financial reasons.