The rule that most affected the economic opportunities of Black South Africans was "Rules forbidding Black South Africans from owning land outside Bantustans."
The establishment of Bantustans (or homelands) was a key component of apartheid policies, which restricted Black South Africans to designated areas and severely limited their rights to own land, access economic opportunities, and work in certain professions. These restrictions had a profound impact on their economic prospects and contributed to systemic poverty and inequality.
While the other options also played significant roles in the broader historical context, the land ownership restrictions were particularly detrimental to the economic wellbeing of Black South Africans.