List these items in order from largest to smallest: organelle, cell, tissue, organism, population, molecule.

Let's reason our way through this.

Organelles are the substructures (such as mitochondria and chloroplasts) inside cells that perform particular functions. They are therefore smaller than cells.

Cells are self-contained self-reproducing pieces of living matter, which form the makeup of all living organisms. They are therefore smaller than organisms.

Tissues are groups of cells that perform a common function, such as skeletal muscle tissue or fat tissue. They are therefore larger than cells.

Organisms are made up of organs which are made up of tissues, and are therefore larger than tissues.

Populations are groups of organisms living in a particular region. They are therefore larger than organisms.

Finally, molecules are the tiniest possible form of a given chemical substance that maintains its chemical properties. Chloroplasts contain molecules of chlorophyll. Therefore molecules are smaller than organelles.

Combine all of these things and this is our ordering from largest to smallest:
Population, organism, tissue, cell, organelle, molecule.