The correct answer is C.
The law of triangle of forces states that if three forces acting on a point can be represented in magnitude and direction, by the sides of a triangle taken in order, these will be in equilibrium.
In other words, if you draw a triangle with the three forces as the sides, and the forces are in equilibrium, then the three sides of the triangle will be in proportion to the three forces.
Here is a diagram to illustrate the law of triangle of forces:
[Diagram of a triangle with three forces acting on a point]
The three forces are $F_1$, $F_2$, and $F_3$. The sides of the triangle are $a$, $b$, and $c$. The law of triangle of forces states that $F_1:F_2:F_3 = a:b:c$.
This law can be used to solve problems involving forces. For example, if you know the magnitude and direction of two forces acting on a point, you can use the law of triangle of forces to find the magnitude and direction of the third force.
The other options are incorrect.
Option A is incorrect because the sides of the triangle do not need to be in proportion to the forces. For example, if the three forces are equal in magnitude, then the sides of the triangle will be all the same length.
Option B is incorrect because three forces acting along the sides of a triangle are not always in equilibrium. For example, if the three forces are not equal in magnitude, then the triangle will not be a right triangle, and the forces will not be in equilibrium.
Option D is incorrect because the resultant of two forces is not always represented by the third side of a triangle. For example, if the two forces are not equal in magnitude, then the resultant will not be in the same direction as the third side of the triangle.