I am not sure but I think the best answer here is C, because "submicroscopic" means "too small to be seen by an ordinary light microscope". And because the name of this aparatus sounds so specifically, the answer B - microscopic - not seems to be the best.
"Scanning Tunnelling Microscopy" (STM) is a technique that has been used for the surface study of NANOSTRUCTURES using the chemical composition of atoms and molecules in NPs to image surfaces at atomic level. - This is the definition of STM. It includes the word NANOSTRUCTURES, so the answer C must be the best one.
