What type of air seal is used in the combustor and turbine mid-frame of a gas turbine?

Correct Answer: Option D — Fishmouth

Fishmouth seals are specialized expansion joints used in high-temperature gas turbine sections like the combustor and turbine mid-frame. Their design accommodates significant thermal expansion and contraction of engine components while maintaining an effective air seal, preventing gas leakage between stationary parts subjected to extreme thermal gradients.