A charged particle moves in a transverse magnetic field. The quantities that will remain unchanged are

$$\begin{gathered} \mathrm{Force} \mathrm{acting} \mathrm{on} \mathrm{a} \mathrm{charged} \mathrm{particle} \\ \mathrm{F}=\mathrm{q}\left(\stackrel{\rightharpoonup }{\mathrm{v}}\times \stackrel{\rightharpoonup }{\mathrm{B}}\right) \end{gathered}$$

so force is perpendicular to velocity work done is zero so KE , speed and magnitude of momentum remains constant.