Answer: The correct answer for the blank is-
amoeba.
Protoplasm is considered as the living part of cell, which includes cell membrane, cytoplasm, and the nucleus.
Amoeba ( it is a protozoan, which is a group of kingdom protista) is the simplest living form of unicellular eukaryotic organisms. It does not have any specified shape. All the cellular activities that are essential for its life, takes place in the protoplasm. As all the properties of protoplasm can be readily observed in this simple organic form ( Amoeba), therefore, it is considered as the mass of living protoplasm.