Step 1: Understanding the Word:
The word means "without interruption; constantly."
Step 2: Analyzing the Spelling:
The correct spelling is derived from the root 'cease' (to stop) and the prefix 'in-' (not). The structure is In-cessant-ly.
Let's check the options:
(A) Incessaently - Incorrect vowel 'ae'.
(B) Incissantly - Incorrect 'i' instead of 'e'.
(C) Incesantly - Missing the second 's'.
(D) Incessantly - This spelling is correct. It has 'in-', 'cessant' (from cease), and '-ly'.
Step 3: Final Answer:
The correctly spelt word is Incessantly.