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
This option means the that pilots avoid expenses of requirements of training. The requirements don't have expenses, the training does.
INCORRECT(B) requirements offered for training by private companies
Same error as in option A
INCORRECT(C) the training requirements of private companies
Same error as in option A
INCORRECT(D) the training offered by private companies
"the training offered by private companies" and "the service required by the Navy or Air Force" are parallel and correctly indicates that pilots avoid expenses of training.
(E) private companies' training
"private companies' training" and "the service required by the Navy or Air Force" are not parallel. "private companies'" is possessive noun and service is normal noun,
INCORRECTHence I chose
option D