Flat capnogram found in A/E –
Disconnection of anesthetic tubing
Accidental extubation
Mechanical ventilation failure
Bronchospasm
All the four options given in the question can cause flat Capnography. But, bronchospasm is the single best answer here as only severe bronchospasm (causing complete respiratory obstruction) can cause a flat capnograph. However, bronchoconstriction is usually partial which causes a steep plateau in capnograph (not flat capnograph).
A flat capnograph is seen in accidental extubation, equipment failure, disconnection (dissociation) of a tube, total occlusion, mechanical ventilation failure, cardiac arrest, severe bronchospasm completely obstructing the airway and complete occlusion of pulmonary circulation.