The skin of which fish is used to prepare “war helmets”?

Correct option is D

Dried porcupine fish with erect spines and hardened body were fashioned historically into helmets in some Pacific island cultures.
Cultural / Functional Details (pointers)
Materials: dried, inflated body with spines provides a rigid and protective outer shell.
Regions: documented use in Micronesia and Gilbert Islands.
Advantage: lightweight, spiky deterrent and symbolic appearance.
Related uses
Shark skin (shagreen) has been used for abrasive surfaces and ornamental leather, not typically helmets.
Notes on other options
Sturgeon: prized for caviar and isinglass; not helmet material.
“Porcupine” here refers to porcupinefish (Diodontidae), not terrestrial porcupine.