What does the presence of 6 molecules of water in this equation indicates to?
This reaction only means complete oxidation of glucose to 6 molecules of carbon dioxide and 6 molecules of water.

Reaction presented in question is very generalized, but the presence of six water molecules only means complete cellular respiration. Check out the actual biochemical pathways which take place to oxidize one glucose molecule.

And other options do not represent the complete cellular respiration, so there will not be formation of six water molecules, only option C means complete oxidation of glucose.


Via: https://en.wikipedia.org/wiki/Cellular_respiration

Water molecules shown by solid blue arrows.

During glycolysis, One glucose molecule releases two water molecules. This is the 9th step  of glycolytic pathway controlled by Enolase. Two water molecules are produced because two reactions are taking place.

(Image source:  https://johncarlosbaez.wordpress.com/2016/01/08/glycolysis/)

During Citric acid cycle, 2 water molecules are used in two enzymatic reactions performed by  Fumarase and Citrate Synthase. And Aconitase performs the  isomerisation of citrate to isocitrate so it has no net effect on water molecules. So, 4 water molecules are used here.


(Image Source: http://www.biosciencenotes.com/citric-acid-cycle/)

During Electron transport chain, 12 water molecules are produced due to oxidation of NADH (FADH2).  

As electrons in this chain are passed, they move from higher to lower energy state, and release energy. Some of the energy is used to transport hydrogen ions across the membrane. This creates electrochemical gradient. And electrons are transferred to molecular oxygen to form water.

This makes net total of 10 water molecules. But 4 water molecules during cellular respiration are used in substrate level phosphorylation. Check out its explanation here   (Via : https://onlinelibrary.wiley.com/doi/pdf/10.1016/0307-4412%2895%2900140-9)



(Image Source: https://www.khanacademy.org/science/biology/cellular-respiration-and-fermentation/oxidative-phosphorylation/a/oxidative-phosphorylation-etc)