The relation between length of sides of triangle and segment's length from vertex to a point on opposite side. It is referred as:
Pythagoras theorem states relation between hypotenuse and oother two sides of a triangle.
Law of tangents states the relation between tangents of two angles of a triangle and the lengths of the opposing sides.
But, Apollonius' theorem states that given a triangle ABC, if D is any point on BC such that it divides BC in the ratio n:m (or mBD=nDC), then
mAB2+nAC2=mBD2+nDC2+(m+n)AD2
Thus, it states the relation between length of sides of triangle and segment's length.
Hence, option C is correct.