The correct answer is B. South Africa brushed off the threat of tougher economic sanctions.
To brush something off means to ignore it or to dismiss it as unimportant. In this case, South Africa is ignoring the threat of tougher economic sanctions from the international community. This is likely because South Africa is confident that it can withstand the economic impact of these sanctions. Additionally, South Africa may believe that the international community is unlikely to impose these sanctions, as they would have a negative impact on the economies of other countries.
The other options are incorrect because they do not accurately reflect the meaning of the sentence. Option A, of, means to remove something from something else. Option C, down, means to knock something down or to lower something in position. Option D, about, means to be concerned with something or to talk about something.