The property which is not of an electromagnetic wave travelling in free space is that :

The correct answer is they originate from charges moving with uniform speed.

An electromagnetic (EM) wave in free space is a solution of Maxwell’s equations when no charges or currents are present. The wave‑equation derived from Maxwell’s equations in vacuum is:

$\frac{\mathrm{\partial ²}E}{\partial t^2}=c^2\nabla ²E$

where $c=\frac{1}{\mu }$₀ε₀ is the speed of light in vacuum. The same equation holds for the magnetic field $B$.

From these equations we obtain several characteristic properties of EM waves in free space:

• They are transverse: the electric field $E$ and magnetic field $B$ are perpendicular to the direction of propagation and to each other.
• The energy density stored in the electric field equals the energy density stored in the magnetic field: $\mathrm{u\_E}=\frac{\epsilon }{}$₀E2 and $\mathrm{u\_B}=\frac{B^2}{\mu }$₀, and for a plane wave $\mathrm{u\_E}=\mathrm{u\_B}$.
• They propagate with speed $c=\frac{1}{\mu }$₀ε₀, a universal constant in vacuum.

The statement “they originate from charges moving with uniform speed” does not describe a property of the wave itself. In vacuum there are no charges to “originate” the wave; the wave can be generated by accelerating charges (e.g., an oscillating dipole), but a charge moving with constant velocity does not radiate electromagnetic energy. According to the Larmor formula, radiation requires acceleration, not uniform motion. Therefore this option is not a property of an EM wave traveling in free space.