Vitamins taken in mega doses can be harmful because
can be stored in the body's fat tissue
can contain bacteria
can cause cancer
can affect the dna

Option). can be stored in the body's fat tissue.

Explanation:

Vitamins are essential micronutrients, required by organisms for the proper functioning of body.

Abnormally high levels of vitamins in the body due to mega doses of vitamins can leads to a condition with toxic symptoms, known as hypervitaminosis. It is caused by storage of vitamins (mainly fat soluble vitamins, such as vitamin D and A) in the fat tissues of body.  

Thus, the correct answer is first option.