A.) Exposure to stomach acid from vomiting
A.) is correct because when bulimics forcefully vomit, stomach acid may also come up along with food. Stomach acid is very acidic, so repeated exposure leads to high chances of enamel and tooth decay. Those suffering from bulimia often permanently damage their throat and mouth.
B.) is incorrect because vitamin deficiency usually doesn't lead to dental issues. However, vitamin deficiency is a common side effect of bulimia. Due to constant purging, the body is not able to absorb the vitamins and minerals from food. Experiencing electrolyte imbalances are also likely as a result of forced vomiting and excessive exercise.
C.) is incorrect because those with bulimia are not necessarily less hygienic. Bulimics are not less likely to brush, floss, or otherwise take care of their teeth. They develop dental issues because normal hygiene practices are not strong enough to counteract sustained acid exposure from purging.
D.) is incorrect. However, it is not uncommon for those with bulimia to develop more cavities as a result of eating this type of food. I believe this is the second best answer because dental issues are usually more likely to be contributed to vomiting. Additionally, constant purging is just as likely to cause cavities and tooth erosion, if not more. On another note, eating food high is sugar is not special to bulimia. Those suffering from many types of eating disorders consume this type of food in higher quantities. Furthermore, those with bulimia do not necessarily always binge on foods high in sugar. While they usually purge after intaking food with high sugar contents, they can purge after all types of meals (typically more calorie-dense meals, but, again, not necessarily).