Option B is kind of an obvious assumption otherwise the whole argument will be meaningless.
Option E might look tempting and would have been the correct choice if the word "some" was used in place of "many".

Conclusion: The number of disabled people who are involuntarily unemployed will drop substantially.

(A) Many congressmen were reluctant to pass the new legislation to prevent discrimination against the disabled. => Out of scope. Congressmen's intention is not related to conclusion.
(B) Some private employers in the past deliberately chose not to hire qualified but disabled job applicants. => Negate this option - Private employers in the past hired disabled applicant. If this is true then disabled applicants will not have new employment opportunities, hence disabled unemployment will not drop. Conclusion falls apart, so this the assumption.
(C) The federal government currently employs more disabled people than does private industry. => Out of scope. Comparison between fg and pi is not related to conclusion.
(D) The approved legislation would stop discrimination against the disabled in the public and private sectors. => Out of scope. Unemployment and not Discrimination is discussed the argument.
(E) Many disabled people voluntarily choose to remain unemployed. => Out of scope. Argument is on unemployment and not discrimination.

HTH

It has been against the law for federal agencies and federal contractors to discriminate against a
qualified job applicant because of a disability - facts/ background information.

Now that Congress has approved legislation to expand these existing provisions to cover private
industry as well - premise/ an action taken

the number of disabled people who are involuntarily unemployed will drop substantially. - conclusion./ prediction of the author based on the premise.

Now the assumption must be corresponding to the prediction - since the law is made for private industry will lead to a decrease in disabled people who are involuntarily unemployed - it must be that the private industry was playing a major role for the large mass of disabled people who are involuntarily unemployed. (B states this)

The other way of thinking is causality :

Congress approving legislation to expand these existing provisions to cover private
industry (cause) ------------------> the number of disabled people who are involuntarily unemployed will drop substantially (effect)

B shows that these two events are interlinked.


Hope this helps !!