This question also ultimately is a contest between two close options, (a) and (c). D can be identified as the opening sentence and mandatory pair (B-C) is also present in all the options. To choose the correct option, think about the flow of the paragraph. It starts with a statement that is broad in scope. Statement D states that apartheid laws institutionalised racial discrimination. If we consider the pair (B-C) however, it is only talking about one specific act, which narrows down the scope of the discussion considerably. Now, we have statement A, which again compared to (B-C) is a little broad in scope as it discusses different aspects which were affected by these laws. Therefore, (D-A) goes much better with the flow of the writing.