Select the INCORRECTLY spelt word.

English spelling errors in words built from a Latin prefix + root + suffix (such as in-/im- + root + -ate/-able) most often come from doubling a consonant that should appear only once, or the reverse. To check such a word, split it into its prefix, root, and suffix, and verify each segment against the word's standard dictionary form.

• Imperturbable = im- (not) + perturb (Latin perturbare, to disturb) + -able: each consonant in the '-turb-' segment appears only once, matching the standard dictionary form.

• Immaculate = im- (not) + maculate (Latin macula, a spot/stain): the prefix 'im-' before a root beginning with 'm' doubles the 'm', giving 'immaculate' as the standard dictionary form.

• Inarticulate = in- (not) + articul- (Latin articulus, a joint) + -ate: the root segment '-articul-' takes a single 'l', so the standard dictionary form is 'inarticulate', not 'inarticullate'.

• Impervious = im- (not) + pervious (Latin per, through + via, way): each consonant appears singly, matching the standard dictionary form.

Cross-checking against related word forms confirms this: 'articulate', 'articulation', and 'articulately' all use a single 'l' in the '-articul-' segment in every standard dictionary; no accepted form of this word family carries a double 'l'.

So the word given as 'Inarticullate' carries an extra 'l' that the standard spelling does not have, making it the incorrectly spelt word among the four options.