HOTSPOT
You develop a webpage. You create the following HTML markup:
The layout of the webpage must contain three rows. The first row spans the entire width of
the page and is labeled #Top. The second row contains three columns. The first column is
labeled #Left; the second column is labeled #Main; and the right column is labeled #Right.
The #Left and #Right columns are a fixed width. The #Main column occupies the remaining
available space. The third and final row spans the entire width of the page and is labeled
#Bottom.
The layout of the webpage must resemble the following image:
You need to create the CSS styles to implement the layout.
How should you complete the relevant styles? (To answer, select the appropriate option
from each drop-down list in the answer area.)
Answer: 
Explanation:
Should be -ms-flex. I don’t think -ms-flexbox even exists.
-ms-flexbox is for IE 10. Since the questions is related to Microsoft -ms-flexbox is right answer.
https://msdn.microsoft.com/en-us/library/hh673531%28v=vs.85%29.aspx
Actually, -ms-flex doesn’t work.
It is either
display:-ms-flexbox; //in IE10
or
display:flex; //IE11 and newer
IE11 and newer versions fully support flex property & you don’t need the -ms-. 9 and older don’t even support this property.
The best practice is to put it all in the code (for other browsers also) because you can’t know which browser the user is using.
The answers are correct.