The lines of force of a uniform magnetic field refers to the imaginary lines that represent the direction of the magnetic field at various points in space. These lines provide a visual representation of the magnetic field`s strength and direction.

Option 1 states that the lines of force must be convergent. This means that the lines would be moving towards a central point. However, this is not true for a uniform magnetic field. The lines of force in a uniform magnetic field are not convergent.

Option 2 states that the lines of force must be divergent. This means that the lines would be moving away from a central point. Again, this is not true for a uniform magnetic field. The lines of force in a uniform magnetic field are not divergent either.

Option 3 states that the lines of force must be parallel to each other. This is the correct answer. In a uniform magnetic field, the lines of force are parallel to each other. This means that they do not intersect or converge/diverge towards or away from any central point.

Option 4 states that the lines of force intersect. This is not true for a uniform magnetic field. The lines of force in a uniform magnetic field do not intersect each other.

Therefore, option 3 is the correct answer.