The employees in our organization,
unlike that in your organization, are forbidden from discussing client names or activities outside the office.
(A) unlike that in your organization, are forbidden from discussing -> "that" is a singular pronoun, we have "employees" is plural, So it is Subject verb agreement issue. Incorrect.
(B) contrasted to ones in your organization, are forbidden to discussing -> "to discussing" is incorrect.
(C) unlike them in your organization, are forbidden to discuss-> "them" is used in objective form, in general after verb. Incorrect.
(D) unlike those in your organization, are forbidden to discuss -> "those" is plural of "that and "those" refers back to "employees". So, comparison seems okay. Further, "forbidden to infinitive" is correct as well. Let's keep it.
(E) unlike the employees in your organization, are forbidden from discussing -> comparison seems okay. Now, Idiom is
"forbidden from x to y"....We don't have "y". Incorrect.
So, I think D.