The correct answer is South African exporters who are foreign buyers of South African goods.
This is because South African exporters typically earn revenue in South African Rand (ZAR) when they sell goods domestically, and while they may transact with foreign buyers, they do not directly supply US dollars. The other options (foreign holders of US dollars buying South African assets, South African importers paying in US dollars, and foreign tourists spending US dollars in South Africa) are all clear sources for the supply of US dollars in the South African economy.