The correct answer is $\boxed{\text{B. 3 kgf, 4 kgf}}$.
Let $F_1$ and $F_2$ be the magnitudes of the two forces. We know that $F_1^2 + F_2^2 = 5^2 = 25$ and $(F_1 + F_2)^2 = 37^2 = 1369$. Solving these two equations, we get $F_1 = 3$ and $F_2 = 4$.
The two forces can be represented by two vectors, $F_1$ and $F_2$, that are at right angles to each other. The resultant force, $R$, is the diagonal of the rectangle formed by the two vectors. The magnitude of $R$ is given by the Pythagorean theorem:
$$R^2 = F_1^2 + F_2^2$$
In this case, $R = 5$, so $F_1^2 + F_2^2 = 25$.
If the two forces are at an angle of 60° to each other, the resultant force can be found using the law of cosines:
$$R^2 = F_1^2 + F_2^2 – 2F_1F_2\cos(60^\circ)$$
In this case, $R = 37$, so $F_1^2 + F_2^2 – 2F_1F_2\cos(60^\circ) = 1369$.
Solving this equation for $F_1$ and $F_2$, we get $F_1 = 3$ and $F_2 = 4$.