Cross-contamination has occurred when:

• A. Microorganisms are unintentionally transferred from one surface or substance to another.
• B. A sterile surface is exposed to air for less than one minute.
• C. A sample is incubated at the optimal temperature for bacterial growth.
• D. A culture medium is prepared using distilled water.

Answer: (A)

Verified step by step guidance

1. Understand the definition of cross-contamination in microbiology: it refers to the unintentional transfer of microorganisms from one surface, substance, or environment to another, which can lead to contamination of sterile materials or samples.
2. Analyze each option to see if it fits this definition: check if the event involves the movement of microorganisms from one place to another unintentionally.
3. For the first option, 'Microorganisms are unintentionally transferred from one surface or substance to another,' recognize that this directly matches the definition of cross-contamination.
4. For the second option, 'A sterile surface is exposed to air for less than one minute,' consider that while exposure to air can introduce microorganisms, the key factor is whether microorganisms are actually transferred, and the time alone does not define cross-contamination.
5. For the remaining options, 'A sample is incubated at the optimal temperature for bacterial growth' and 'A culture medium is prepared using distilled water,' note that these describe normal laboratory procedures and do not involve unintentional transfer of microorganisms, so they do not represent cross-contamination.