Answer:
The answer is actually "Elaborate vocabulary".
Step-by-step explanation:
Basically what it is saying is that you need accurate spelling, accurate punctuation, and accurate grammar for written communication. All of those are important for the reader's comprehension. But if you have an elaborate vocabulary in the letter, the reader might actually struggle with comprehension. For example:
To Mr. Icantunderstandz
Hello dear friend, I write you from my paradisaical edifice while I sip on the moreish English tea which you have gifted to me. I have been inundated with speculations of your getting engaged to a lovely young sheila. Please do initiate me on your wellness and the particulars of your life. I would love to acquire more on the subject of your results.
- Mr. tomanywordz
As you can see, this is not understandable to most people. with that in mind, It is best to keep vocabulary basic; but not to simple.