Final answer:
Soy milk is the best source of vitamin B-12 for a vegan diet from the provided options, as it is typically fortified with this essential nutrient.
Step-by-step explanation:
The best source of vitamin B-12 for a vegan diet among the options provided is soy milk. Vitamin B-12 is essential for a wide variety of cellular processes and is normally found in foods of animal origin. Since vegans avoid animal products, fortified foods like soy milk, as well as certain cereals, can be excellent sources of this vitamin. Cottage cheese and broiled salmon are rich in vitamin B-12 too, but they are not suitable for a vegan diet as they are animal-based products. Therefore, fortified foods such as certain breakfast cereals or plant milks, like soy milk, are recommended for individuals following a vegan diet to meet their vitamin B-12 needs.