Explanation

The very first thing to do is to dive into the gist of the problem, or the core, trying to get the real meaning of the sentence; no matter what the complexity of what it is.

Now, the first part of it before the comma, actually, is directing us to what he “proved” is actually something “nobody would think of denying”, which means in a simple gergo: Romero proved something that is irrefutable or thing like that.

After the comma, Romero wrote something with the characteristics of completeness. Honestly, there are not so many clues along the sentence BUT if someone write at an advanced stage of his/her life something that is difficult to disprove. This, in general, is the result of the experience and sagacity - this should lead us to think of something similar in the answer choices.

A) Maturity - maybe we are lucky enough of finding immediately the probable solution. Keep it for now
B) Fiction - we do not have so many info about what he wrote or else. Cross off.
C) Inventiveness - here we are talking about Romero and his writing as a whole in his late stage of life. Nothing suggests us whether his writing is fancy or else.
D) Art - this word does not fit very well with the characteristics.
E) Brilliance - this could be the answer but it is quite similar to the meaning-trap of art. No good.

A is the best of the bill.

But the first paragraph says "distinction" does that not imply exceptional talent distinction " ?

Honestly, I do think the very first words are a distraction. For me should be "dubious" which means questionable but exactly the contrary: undubituous.

Either way, try to take the overall meaning of the sentence as a whole. Otherwise, you could fall into a trap.

Hope this helps

Just because no specific word in the sentence is stressed, the age of the author is taken as clue.

The word dubious plays an important role here because, without it, one can easily choose the word inventiveness. The word distinction could have become a clue here without the word dubious. But if the distinction is dubious, then it should be something other than being inventive, so the word mature is relatively applicable here.
_________________

It is his dubious distinction to have proved what nobody would think of denying, that Romero at the age of sixty-four writes with all the
characteristics of (i) _______

Blank (i)
maturity
fiction
inventiveness
art
brilliance

The key here is "what nobody would think of denying". So, the correct option would be something that a sixty four year old would obviously have. The best fit is maturity.

"Dubious" is definitely a trap.
_________________

It is his dubious distinction to have proved what nobody would think of denying, that Romero at the age of sixty-four writes with all the characteristics of (i) ___________.

A dubious distinction is a distinction that you should not be really proud of.

Now we are told that his dubious distinction was proving what nobody would think of denying. In other words, he proved the obvious. And the truth in the following clause

"that Romero at the age of sixty-four writes with all the characteristics of (i) ___________."

becomes obvious when we choose maturity for the blank.

It should obvious that a man of sixty-four writes with all the characteristics of a sixty-four year old. And among the list of choices, the only word which describes the characteristics of a sixty-four year old is maturity.
_________________

This is how I solved
what would no one think of denying - that romero (older person) writes with maturity