I have a hunch it's E, even though you forgot the length of the side.
1 is insufficient because we cannot find the sum of sides with only one side
2 is insufficient because we do not have any other clue except for the fact that two sides are equal.

Taken together, they are still not enough, since you can't tell if the length of the equal sides is the value provided in stmt 1 or if said length is actually the other side. However, I might be wrong and maybe the length provided in 1 is useful, by applying the rule that a + b > c (lengths of sides).

Edit: I missed the "right" part of the question. Say a = length of equal sides. This means hypotenuse = a*sqrt(2). Since area = (a^2)/2, then a^2 = 50, with a = 5*sqrt(2). This means perimeter = 2*5*sqrt(2) + 10 = 10sqrt(2) + 10. So B would be the answer.

naaga wrote:If the area of triangular region RST is 25, what is the perimeter of RST?
(1) The length of one side of RST is .
(2) RST is a right isosceles triangle.

(1) Knowing the length of one side says nothing since we can make an infinite amount of triangles with an area of 25 knowing the length of one side.

(2) If the triangle is a right-isosceles, and we know the area is 25, there can only be ONE triangle that fits this description. The angle of the triangle is right, meaning the other two angles are 45 degrees. Knowing this, we know that there is EXACTLY ONE triangle that can be made the with known area of 25.

Thus (B)

yes the OA is ( B )