Please find the attached file for a proper understanding of the question given here. The diagram in the file attached is the complete accompanying diagram of the question.
As per that diagram, [tex] RS\cong SB [/tex] (which is given)
and we know that [tex] \angle DSB\cong \angle RST [/tex] (vertically opposite angles).
Thus, the only way ΔDSB ≅ ΔTSR by SAS is when [tex] DS\cong ST [/tex] as indicated in the diagram.
Thus, [tex] DS\cong ST [/tex] is the correct answer.
