Which is water soluble vitamin? (a) Niacin (b) riboflavin (c) ascorbic acid (d) trypsin?

Trypsin (EC 3.4.21.4) is a serine protease from the PA clan superfamily, found in the digestive system of many vertebrates, where it hydrolyses proteins.
en.wikipedia.org/wiki/Trypsin

Vitamin C, also known as ascorbic acid and L-ascorbic acid, is a vitamin found in food and used as a dietary supplement. As a supplement it is used to treat and prevent scurvy.[1]
en.wikipedia.org/wiki/Vitamin_C

Riboflavin, also known as vitamin B2, is a water soluble dye that makes urine bright yellow. When the color fades, it is time for another dose.
~ "Let's Eat Right" by Adelle Davis

Niacin, also known as nicotinic acid, is an organic compound with the formula C6H5NO2 and, depending on the definition used, one of the 20 to 80 essential human nutrients. Together with nicotinamide it makes up the group known as vitamin B3 complex.[2]
en.wikipedia.org/wiki/Niacin

Since niacin and vitamin C are acids, they are water soluble by definition. Trypsin is not called a vitamin.