Be careful when reading instructions: check whether you need to find the 'correctly' spelled word or the 'incorrectly' spelled word.
The suffix '-ure' in 'adventure' is often misspelled with an added 'o' due to confusion with phonetic sounds.
Step 1: Identify the incorrectly spelled word. We need the one option that contains a spelling error. Step 2: Check each option. Option 1 - innocuous: correct. Option 2 - prodigious: correct. Option 3 - heterogenous: correct. Option 4 - adventoure: incorrect; the correct spelling is adventure (no extra o). Step 3: Confirm. The extra vowel o inserted in adventoure makes it the misspelled word in this group. \[ \boxed{\text{adventoure}} \]