You want to go from the BIG triangle to the little one.

Base length of small triangle = s
Base length of big triangle = S
Height of small triangle = h
Height of big triangle = H

Both triangles are similar, i.e. their respective internal angles are the same. Therefore:

h/H = s/S ... (equation 1)

Area of big triangle = 2 x Area of small triangle
(1/2)*H*S = 2*(1/2)*h*s
H*S = 2*h*s
S = 2*(h/H)*s (equation 2)

Inserting equation 1 into equation 2:

S = 2*(s/S)*s
S^2 = 2*(s^2)
S = s*sqrt(2)

Choose C.

-BM-