Directions: In the following sentence, four words, marked as A, B, C, and D, are given in bold, out of which one word may or may not be misspelt. Identify the misspelt word and mark it as your answer. If all the highlighted words are correctly spelt, mark "All are correctly spelt" as your answer.

The detective’s acumen (A) and astute (B) observations, combined with his dogged (C) determination, led to the resolution of the intractible (D) case.