C

Step 1: Consider the three forces
F1 = 5 N, F2 = 4 N, and F3 = 3 N.
Step 2: Use the Law of Cosines to relate the forces and the angle θ between F1 and F2.
The equation is: F1^2 = F2^2 + F3^2 - 2 * F2 * F3 * cos(θ).
Step 3: Substitute the known values: 5^2 = 4^2 + 3^2 - 2 * 4 * 3 * cos(θ).
25 = 16 + 9 - 24 * cos(θ).
25 = 25 - 24 * cos(θ).
Step 4: Rearrange to find cos(θ): 0 = -24 * cos(θ)
This implies θ = 60° (θ = 180° - 60°) is one solution, and the angle between force F1 and F2 is 60°
Step 5: Check the other relationships to verify mutual angles between F3.
Therefore, the angle is 60°.