Where $\;0.5 A^{0}\;$ x-rays strike a material , the photoelectrons from K shell are observed to move in a circle of radius 23mm in a magnetic fields of $\;2\times10^{-2}T\;.$ Than the binding energy of K-shell electrons is

Question

$(a)\;1.3 KeV\qquad(b)\;4.2 KeV\qquad(c)\;5.8 KeV\qquad(d)\;6.2 KeV$