Final answer:
The action taken by the researchers that makes the study a randomized controlled trial is that they do not know which patients will be treated with saline and which patients will be treated with chlorhexidine.
Step-by-step explanation:
In this study, the action taken by the researchers that makes it a randomized controlled trial is option a. The researchers do not know which patients will be treated with saline and which patients will be treated with chlorhexidine. In a randomized controlled trial, the treatment assignment is random and not based on any previous knowledge or bias.