Final answer:
The nursing assistant knows how to prepare the solution correctly when she states Option B: 'I put the castile soap in the enema bag first, then I add 250 mL of fluid.'
Step-by-step explanation:
The nursing assistant knows how to prepare the solution correctly when she states Option B: 'I put the castile soap in the enema bag first, then I add 250 mL of fluid.' This is the correct procedure for preparing the solution. By adding the castile soap to the enema bag before adding the fluid, the soap will mix properly with the fluid and create the desired solution.