which of the following pairs have identical value of e/m? (a)a proton and a neutron (b)a proton and deuterium (c)deuterium and an alpha particle (d)an electron and gamma rays 

You are correct.

(c) is the correct answer deuterium and an alpha particle has identical e/m ratio.

They both emit and absorb at the same frequencies. The e/m ratio of alpha particle is 4.82245111 x 10^7 C/kg which is identical with deuterium.