IF IT IS SMALL THEN WE USE MICROSCOPE .WHEN IT IS VERY SMALL ORGANISM THEN WE USE ELECTRON MICROSCPOE
If it is small to see , we can use magnifying glass but if it is micro organism , we can see only through a microscope.
Guys! Are you crazy? microscope is used to see microorganisms.barometer is used to measure the amount of rain.
First of all , many have answered it vaccum but it is absolutely wrong because sound can not travel in vaccum and most probably the correct answer will be solid.