Final answer:
When discarding a used pipette, it should be put in the designated pipette bin.
Step-by-step explanation:
When discarding a used pipette in a plastic benchtop pipette discard bin, the pipette should be put in D) The designated pipette bin.
The reason for this is that pipettes used in labs may contain hazardous chemicals or biological agents, so they should not be disposed of in regular trash or recycling bins. Instead, they should be placed in a designated pipette bin.
It is important to follow proper disposal procedures to ensure the safety of both the environment and individuals.