Final answer:
The instrument that counts particles based on their relative size and density is the impedance counter. It is an electronic device that counts cells by measuring the change in resistance in an electrolyte solution when a cell passes through a small opening in a container wall.
Step-by-step explanation:
The instrument that counts particles based on their relative size and density is the impedance counter. An impedance counter is an electronic device that counts cells by measuring the change in resistance in an electrolyte solution when a cell passes through a small opening in a container wall. The detector automatically counts the number of cells passing through the opening. This method allows for accurate measurement and counting of particles based on their size and density.