Suppose a charged particle is projected inside a gravity-free room...

The force on a moving charge $q$ with velocity $\vec{v}$ in an electric field $\vec{E}$ and magnetic field $\vec{B}$ is given by the Lorentz force:

$\vec{F} = q(\vec{E} + \vec{v} \times \vec{B})$

For the particle to experience a deflection, the net force perpendicular to its velocity must be non-zero.

If both $\vec{E}$ and $\vec{B}$ are zero, the net force is zero, and the particle will move in a straight line without any deflection. Hence, the two fields cannot both be zero.

A deflection can be caused by an electric field alone (if $\vec{E}$ has a component perpendicular to $\vec{v}$) or by a magnetic field alone (if $\vec{B}$ has a component perpendicular to $\vec{v}$). Thus, neither a magnetic field nor an electric field is necessarily present.

Both fields can also be non-zero simultaneously and produce a net force that causes deflection.