The correct answer is letter D):when you are adding a suffix that begins with a vowel.
We generally drop the final "e" from a root word as long as the suffix starts with a vowel. For instance, when we add "-ing" , as in write/writing or when we add "-er" as in nice/nicer.
Although there are some exceptions to this rule , as for example the words that end in"-ce" before the suffix "-able" as in notice/noticeable, the general rule is the one mentioned before.
