For both triangles to be similar, the corresponding sides of both triangles are always in the same ratio.
So for the corresponding sides:
1) BA ans SR, the ratio will be
2)AC and RT, the ratio will be
3) BC and ST, the ratio will be
As mentioned above, the ratio between corresponding sides is always the same so that, if the triangles are simmilar the we know that:
The correct option is the first one.
whenever we run a perpendicular line to a base from a right-angle in a right-triangle, like in this case, we end up with 3 similar triangles, a Small one, a Medium and a containing Large one, therefore Check the picture below.