why not C?

C is pretty close.

Reconnaissance means inspection, something that detectives would do. It also means preliminary research. The sentence is saying that the painter uses watercolor to draw sketches - and this tells us that it was done at the preliminary stage. Hence D is the best choice.

Synthesis means combining different ideas or concepts. Having the word "gathering" can fool anyone here. But, the overall meaning of the idea is that watercolor is used to draw an outline of a sketch. Something that is done at the beginning. It doesn't mean you are combining different ideas or things rather it means you are trying to get the fuller understanding of the subject.

I hope it makes sense now!

Cezanne's delicate watercolor sketches often served as ............ of a subject, a way of gathering fuller knowledge before the artist's final engagement of the subject in an oil painting.

Here, Comma is supportive
__________, gather information( keyword from 2nd part)
So blank = gather information


From option
Reconnaissance = gain information

So D) Reconnaissance is ans

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