1. A significant proportion of today’s population certainly prefers buying ready-made food instead of cooking with fresh ingredients.
Notice that "prefers" is correct instead of "prefer," because the subject (proportion) is singular.
2. In addition, the wide range of junk food offered by supermarkets and fast food restaurants, along with the lack of knowledge by the consumer, has caused this national issue.
Notice that the subject is actually "range," which is singular, so "has" is correct (i.e. the wide range has caused...).