yass20015 wrote:Why this one is correct?

The artist Pierre-Auguste Renoir's last word was "flowers," spoke as a bouquet consisting of roses just picked from his garden were arranged in a vase on his bedroom windowsill.

A) as a bouquet consisting of roses just picked from his garden were arranged
B) as a bouquet of roses, just picked from his garden, were arranged
C) as a bouquet of roses just picked from his garden was being arranged
D) during the arrangement of a bouquet of roses, just picked from his garden
E) while they arranged a bouquet of roses that had just been picked, from his garden

There are two things that make the OA, C, correct.

One is that bouquet is a singular noun, modified by of roses, and so was is the correct verb form to go with bouquet.

In addition, were arranged, or was arranged, does not as effectively express what was going on as does was being arranged. The point is not that the bouquet was sitting there arranged. It's that the bouquet was being arranged as he said "flowers".

yass20015 wrote: Why this one is correct ? thanks

You are asking the wrong question.
The OA Is correct because the other answer choices are INCORRECT.
A far better question would be as follows:
Why are the other answer choices incorrect?
I posted a full explanation for this SC here:
https://www.beatthegmat.com/sc-t279365.html