Public Charters under this part shall meet the following requirements:

(a)-(b) [Reserved]

(c) If the charter is on a round-trip basis, the departing flight and returning need not be performed by the same direct air carrier.

(d) The air transportation portion of the charter must be performed by direct air carriers that hold authority under Chapter 411 and 413 of 49 U.S.C. Subtitle VII, or are operating under 14 CFR part 298, except that only U.S. citizen direct air carriers may provide air transportation for operations in interstate air transportation.