Which one of the following is a water soluble vitamin, that is not excreted easily?

We need to identify the water-soluble vitamin that is not easily excreted from the body.

B-complex vitamins and Vitamin C are water-soluble vitamins. Most water-soluble vitamins are readily excreted through urine when present in excess, as they dissolve in water and are not stored significantly in the body.

One exception is Vitamin $$B_{12}$$ (Cobalamin), which, although water-soluble, is stored in the liver in significant amounts (the body can store several years' worth), is not easily excreted from the body, and has a very efficient enterohepatic circulation (reabsorption from bile).

By contrast, Vitamin $$B_1$$ (Thiamine), Vitamin $$B_2$$ (Riboflavin), and Vitamin $$B_6$$ (Pyridoxine) are all water-soluble vitamins that are readily excreted in urine when consumed in excess and are not stored in large amounts.

Therefore, Vitamin $$B_{12}$$ is the water-soluble vitamin that is not easily excreted, as it is stored in the liver and has efficient reabsorption mechanisms.

Therefore, the correct answer is Option D.