In a DATA step concatenation in SAS, when multiple datasets are listed in a SET statement, SAS concatenates the datasets vertically, stacking them one on top of the other. The length of variables in the resulting dataset is determined by the first dataset that appears in the SET statement. If the same variable appears in multiple datasets, SAS uses the length as it is first encountered. It does not require columns with the same name to be renamed; rather, it stacks them directly. There is no maximum number of tables that can be listed in a SET statement for concatenation; more than two can be concatenated. Lastly, concatenation combines data vertically, not horizontally as option D suggests.
References
* SAS 9.4 Language Reference: Concepts, "Concatenating Data Sets."