Answers A, B, and C all are non-factitious assertions. However, answer D is true. Unlike the other three answers, it has proof to it. If you sterilize equipment, therefore removing harmful bacteria from the equipment, the said equipment is less likely to become contaminated.
The question asks which answer shows an example of a true scientific theory.
Answer D is the only answer that complied with that rule set.
In conclusion, the answer is D.
