Answer: D
Correct answer: Definitely
The correct spelling is definitely. There is no a anywhere in it, and the vowel that so often appears as one is an i. The best safeguard is to see the word finite inside it: de-finite-ly. Something definite has been marked out with limits, from the Latin finis, an end - the same root as finish, final, infinite and define. The second error the options offer is dropping the e before -ly. English keeps it: the adjective definite simply takes -ly. The same holds for completely, extremely and immediately, while truly, duly and wholly are the exceptions that drop it. It is consistently among the most misspelt words in English, and the finite test disposes of every variant at once.