This is a TOUGH question (unless you know about architecture!)
Fortunately, there are many contextual clues to help us.
First, the word ARCHITECTURAL tells us that the missing word has something to do with architecture.
Second, we're told that the missing word was added to provide ADDITIONAL SUPPORT. So, the missing word has something to do with providing support.
Let's predict SUPPORTIVE THINGY for the blank....

A) ballast: heavy material used to improve stability
Not bad...KEEP

B) albatross: large bird...ELIMINATE

C) hallmark: a mark of goodness/impurity.
Nothing to do with architecture or support...ELIMINATE

D) buttress: a source of defense or support.

E) trademark: something that indicating legally registered...ELIMINATE.

We're left with BALLAST and BUTTRESS.
Buttress seems like a better fit.

Answer: D

Cheers,
Brent

The famous Notre Dame cathedral in Paris took almost two hundred years to complete; this immense architectural effort included the first notable use of a flying ________, but this renowned feature was not part of the original design and only employed when the walls forming the nave began to crumble and needed additional support.
Notice that the sentence marked in Blue tells us that something was required for 'Support'.
Here, in the choices given, Buttress means to support.
So the answer is D.Buttress.

D ) Buttress - means support

The famous Notre Dame cathedral in Paris took almost two hundred years to complete; this immense architectural effort included the first notable use of a flying ________, but this renowned feature was not part of the original design and only employed when the walls forming the nave began to crumble and needed additional support.

Buttress is the correct answer. All others are way off the mark, especially hallmark!

A buttress is an architectural structure built against or projecting from a wall which serves to support or reinforce the wall.

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