Vitamin D occurs naturally in some foods, such as salmon and Shiitake mushrooms. You can get vitamin D from exposure to sunlight, but to do this in a sensible way, you would need to consciously go outside and expose your arms and legs to 5 to 10 minutes of sunshine, then go back inside and apply your sunscreen for the day. Since most people would have trouble sticking with this routine, it may be better to have some salmon or tuna salad for lunch.
What if you don't like fish or you've had your fill of Shiitake mushrooms and egg yolks? Are there any other options? Of course there are. You could eat foods that have vitamin D added.
