The coffin corner refers to:

Maximum takeoff weight limitation

The narrow speed margin at high altitude where low-speed stall buffet and high-speed Mach buffet converge

Correct

A structural limitation

An engine failure scenario

Explanation

At very high altitudes, the margin between stall speed and MMO narrows; exceeding either limit causes buffet — this narrow band is the coffin corner.

Test yourself with random questions from Principles of Flight and all other ATPL subjects.