A scientist eliminated all living organisms in a jar of soup by heating it. He then left the soup jar uncovered for some days. After a few days, he saw molds in the soup. What most likely formed the mold?

Spores in the air or contamination through some sort of insect interaction. In an open system, there are billions of microbial agents floating around. With an open container containing a food source and time, the likelihood of contamination is nearly %100.