Final answer:
The potentially harmful substance required by law to be identified on a food package label among the given options is trans fat, due to its association with heart disease.
Step-by-step explanation:
Among the substances listed, trans fat is a potentially biologically harmful substance that is required by law to be identified on a food package label. The inclusion of trans fat information on nutrition facts labels became mandatory as of January 1, 2006, due to its association with increased risk of heart disease.
Trans fats are known to raise levels of LDL cholesterol (often referred to as 'bad cholesterol') in the blood, which is a risk factor for heart disease. Other nutrients such as Vitamin C, calcium, and fiber are not harmful but are listed on nutrition labels to help consumers understand their dietary intake of essential nutrients.