A student carried out an investigation to identify the presence of microbes in water from different sources - HSC - SSCE Biology - Question 16 - 2012 - Paper 1

Based on the observations in the investigation, it can be inferred that option (D) is correct: The agar plates were contaminated prior to the beginning of the experiment.

This inference is drawn from the fact that the control agar plate showed microbial growth, indicating contamination. Since the control should not show any growth, it suggests that the sampling process or the agar plates themselves were compromised before the experiment commenced.