In human nutrition, which of the following vitamins are water soluble and therefore included in the category of water soluble vitamins?

Introduction / Context:
Understanding which vitamins are water soluble and which are fat soluble is important for nutrition, storage, and toxicity considerations. This question checks if you know that vitamin C and most B complex vitamins are water soluble, and asks you to select the correct combination among the options.

Given Data / Assumptions:
• Vitamin C, riboflavin (vitamin B2) and niacin (vitamin B3) are listed individually.
• One option is all of the above, and another vitamin, vitamin K, is included as a separate choice.
• We must recall the standard classification of vitamins into water soluble and fat soluble groups.

Concept / Approach:
Water soluble vitamins include vitamin C and all B complex vitamins, such as thiamine, riboflavin, niacin and others. These vitamins are not stored extensively in the body and excess amounts are usually excreted in urine. Fat soluble vitamins include vitamins A, D, E and K, which can be stored in body fat. Therefore, vitamin C, riboflavin and niacin are all water soluble, while vitamin K is fat soluble, so among the options, all of the above referring to vitamin C, riboflavin and niacin is correct.

Step-by-Step Solution:
Step 1: Recall that vitamin C is also called ascorbic acid and is a classic example of a water soluble vitamin. Step 2: Remember that riboflavin, vitamin B2, is part of the B complex group, all of which are water soluble. Step 3: Note that niacin, vitamin B3, is another B complex vitamin and is water soluble as well. Step 4: Recognise that vitamin K belongs to the fat soluble group of vitamins, along with vitamins A, D and E. Step 5: Therefore, vitamin C, riboflavin and niacin are all water soluble, making the option all of the above the correct choice.

Verification / Alternative check:
Nutrition charts and medical references classify vitamins C and B group as water soluble vitamins, explaining that they need regular dietary intake and that excess amounts are usually excreted. In contrast, vitamin K is grouped with fat soluble vitamins which can accumulate in the body. This classification confirms the reasoning used for the question.

Why Other Options Are Wrong:
Vitamin C: Correctly water soluble, but this option ignores the fact that riboflavin and niacin are also water soluble.
Riboflavin only: Leaves out vitamin C and niacin, both of which are water soluble, so it is incomplete.
Niacin only: Again, it omits vitamin C and riboflavin, so it is not the best answer.
Vitamin K: This vitamin is fat soluble, not water soluble, so it cannot represent the entire group of water soluble vitamins listed in the question.

Common Pitfalls:
Students sometimes memorise vitamins without clearly separating water soluble and fat soluble groups. Another mistake is to think that only one of the B vitamins is water soluble. To avoid confusion, remember a simple rule: vitamin C and all B complex vitamins are water soluble, while vitamins A, D, E and K are fat soluble.

Final Answer:
Among the given options, All of the above is correct because vitamin C, riboflavin and niacin are all water soluble vitamins.