Final answer:
Mixing 5.00 mL of an aqueous solution with 45.00 mL of water is an example of dilution, which decreases the solution's concentration. Therefore, the correct option is A.
Step-by-step explanation:
The process described in the student's question, wherein 5.00 mL of an aqueous solution is mixed with 45.00 mL of water, represents a dilution. During dilution, the concentration of the solution decreases because the number of moles of the solute remains constant while the volume of the solution increases, leading to a lower molarity. An equality can be set up to show that the moles of solute before the dilution (mol₁) are equal to the moles of solute after the dilution (mol₂). Therefore, the original solution has been diluted by one-fifth, as the new volume is five times as great as the original volume, resulting in the molarity being one-fifth of its original value.