Which regulation does not prohibit SVFR helicopter operations?

The correct choice allows for Special VFR (SVFR) helicopter operations because 14 CFR Part 91 outlines general operating rules for aircraft that do not impose prohibitions on SVFR flights. This regulation enables pilots to operate in airspace under Visual Flight Rules (VFR) even when visibility is less than the standard minimums, provided that they receive explicit clearance from air traffic control.

In contrast, the other regulations pertain to specific types of operations. Part 93 primarily governs the use of certain airspace within Class B, Class C, and other specified airspaces, but it doesn't relate directly to SVFR procedures. Part 121 applies to scheduled air carrier operations, which operate under stricter visibility and communication requirements that would typically preclude SVFR. Part 135, which relates to commuter and on-demand operations, also has stringent operational criteria not aligned with the flexibility offered by SVFR. Thus, Part 91 remains the regulation that supports helicopter SVFR operations without imposing prohibitions.