Concept:Cellular organisation progresses from cell → tissue → organ; organs are more complex than tissues or single cells.Explanation:Euglena is a single-celled organism, so its organisation is at the cellular level.Hydra has tissues but no true organs, so it is at the tissue level.Hair is a structure made of dead cells and keratin, at the tissue level.The heart is an organ made of different tissues working together, so it is the most complex level of organisation listed.Answer:A. Heart