Recall the general rule

Vitamins are of two types:

• Water-soluble vitamins: B-complex vitamins and vitamin C
• Fat-soluble vitamins: A, D, E, K

Water-soluble vitamins are generally readily excreted in urine and are not stored much in the body.

Identify the exception

Among the water-soluble vitamins, Vitamin $B_{12}$ is a well-known exception because it is:

• stored in the liver in significant amounts,
• therefore not excreted easily compared to other water-soluble vitamins.

Check the options

• A: Vitamin $B_2$ — water-soluble, generally excreted easily
• B: Vitamin $B_1$ — water-soluble, generally excreted easily
• C: Vitamin $B_6$ — water-soluble, though some storage may occur, it is not the standard exception asked here
• D: Vitamin $B_{12}$ — water-soluble and not excreted easily

Conclusion

Therefore, the correct answer is: $\boxed{\text{Vitamin } B_{12}}$