Food handlers can contaminate food when they:
a. keep fingernails short for easy cleaning underneath
b. change gloves for fresh gloves between tasks
c. have been diagnosed with a foodborne illness
d. drink from a closed beverage cup with lid and straw

c. have been diagnosed with a foodborne illness

Explanation:

The only possible option for food contamination is option C, where the person handling the food can contaminate the food depending on the disease that is present and how it is exposed to food. As for the other options, they are valid methodologies to avoid possible contamination of food, since it is short nails to facilitate washing, the replacement of gloves by performing different tasks or drinking liquids from a closed glass with sorbet.