Select the INCORRECTLY spelt word.

English commonly confuses the adjective-forming suffixes "-able" and "-ible": "-able" typically attaches to Latin first-conjugation ("-are") verb stems or free-standing English words (dispensable, indomitable, agreeable), while "-ible" attaches to other Latin verb stems (audible, credible, visible). Because the two suffixes sound identical in speech, only the dictionary spelling of each individual word — not pronunciation — settles which one it takes.

• Interrogate — derived from Latin interrogare with the regular "-ate" verb ending; spelled correctly.

• Indispensible — its root verb is "dispense" (Latin dispensare, an "-are" stem), so the adjective must take "-able"; the dictionary-correct form is "indispensable", so this rendering wrongly substitutes "-ible" for "-able".

• Insouciant — a French loanword retained with its original French spelling; spelled correctly.

• Indomitable — built from in- + domitable with the regular "-able" suffix; spelled correctly.

Cross-checking each word against Merriam-Webster, Oxford, and Collins confirms the same picture: "indispensable" is the only dictionary-recognised spelling, while "indispensible" is a widespread but nonstandard variant; the other three words match their dictionary entries exactly.

So the incorrectly spelt word among the options is "Indispensible" (its correct spelling is "indispensable").