The correct answer is: C. sometimes curved.
The lines of force due to charged particles are sometimes curved. This is because the electric field is a vector field, which means that it has both a magnitude and a direction. The direction of the electric field is from positive charges to negative charges. The magnitude of the electric field is proportional to the charge of the particle and inversely proportional to the square of the distance from the particle.
When two or more charged particles are close together, the electric fields from each particle will interact with each other. This can cause the lines of force to curve. For example, if two positive charges are close together, the lines of force will be curved away from each other. This is because the electric fields from each charge are pushing away from each other.
The lines of force can also be curved by the presence of a conductor. A conductor is a material that allows electricity to flow through it easily. When a conductor is placed in an electric field, the electric field will cause the charges in the conductor to move. This movement of charges creates an electric current. The electric current will cause the lines of force to curve.
In conclusion, the lines of force due to charged particles are sometimes curved. This is because the electric field is a vector field and the presence of conductors can cause the lines of force to curve.