Option 3

It prevents the contamination of the sample with other microorganisms, which could inhibit the growth of the causative agent. 

Dr. Spencer: I see how you would think that preventing contamination of the sample is important in aseptic technique. Preventing contamination is important. Contamination does not necessarily mean, though that the contaminating microorganisms would inhibit the growth of the causative agent. There is another reason why preventing contamination is important. How about you give this another try?

Option 1. It prevents changes in the pH of the growth medium, ensuring that the culture medium remains stable.

Option 2. It minimizes the risk of contamination, allowing for accurate and reliable results by preventing growth from unwanted microorganisms.