The word that completes the sentence is organs.

The biological hierarchy

In living organisms, especially multicellular ones, there is a clear structural hierarchy:

  • Cells are the basic units of life; many similar cells working together form a tissue.
  • Different types of tissues combine to form a structure called an organ , which performs a specific function (like the heart, stomach, or liver).
  • Multiple organs then work together as an organ system (such as the digestive system or circulatory system), and organ systems together make up the whole organism.

So the full sequence is:
Cells → Tissues → Organs → Organ systems → Organism.

Why “organs” fits

The sentence “Cells form tissues. Tissues form __________.” is describing this standard biological organization.

Since tissues join together to create structures like the heart, lungs, kidneys, etc., the correct missing word is organs.

Common mistake to avoid

A frequent mix‑up is saying “tissues form cells” or “cells form organs” directly, but that’s backwards or skips a level.

The correct order is: cells make tissues, and tissues make organs.

Bottom line:
“Cells form tissues. Tissues form organs.”