Final answer:
The yellow plate indicates the presence of coliform bacteria, as coliforms ferment lactose and cause a pH indicator to change color from red to yellow.
Step-by-step explanation:
If we are considering a culture where plates are used to identify coliforms, the plate that turns yellow upon cultivation would signify the presence of coliform bacteria. This is because coliforms are gram-negative rod bacteria that can ferment lactose, which is often included in the medium for coliform testing. When coliforms ferment lactose, they produce acids leading to a pH change in the medium. This change in pH can be detected by a pH indicator in the medium, which, as in the case presented, would change from red to yellow. Therefore, the plate that has turned yellow indicates the presence of coliforms.