Which fats result when manufacturers add hydrogen to fat molecules in vegetable oil?

The correct answer is trans-fat.

Explanation:

The trans-unsaturated fatty acids or trans fat is a kind of unsaturated fat, which takes place in small concentrations in nature, however, it became broadly generated industrially from vegetable fats beginning in the 1950s for the application in snack food, margarine, and others.  

The fats constitute long chains of hydrocarbons that can be either saturated or unsaturated. In food production, liquid cis-unsaturated fats like vegetable oils are hydrogenated to generate saturated fats that exhibit more desirable physical characteristics.