Sentence Correction

The National Labor Relations Act expressly forbids unions from engaging in secondary boycotts against companies not directly involved in a labor dispute.
(A) unions from engaging in
(B) the engagement by unions of
(C) unions to engage in
(D) unions from becoming engaged with
(E) that unions engage upon

I chose A but the OA is C, can someone explain me why?

Please hide OA with spoiler

Here Idioms are tested

forbid X to do Y
prohibit X from doing Y

Only C uses correct idiom

forbid X to do Y
prohibit X from doing Y

one more for C