Sentence Correction

Commercial pilots receive their training from regional airlines with increasing frequency, often preferring to avoid the expense of the requirements of training offered by private companies and the service required by the Navy or Air Force.
(A) the requirements of training offered by private companies
(B) requirements offered for training by private companies
(C) the training requirements of private companies
(D) the training offered by private companies
(E) private companies' training

OA is D
What could have made the OA the best pick here. Olease i need an Expert reply.