The missing word is small. The complete sentence is:
“Small substances can move into or out of cells by diffusion.”

Why “small” fits

  • Diffusion works best for small, non‑charged molecules (like oxygen and carbon dioxide) because they can easily pass through the phospholipid bilayer of the cell membrane.
  • Larger or charged substances usually need facilitated diffusion (with channel or carrier proteins) or active transport , rather than simple diffusion.

Quick comparison

Type of substance| Can it diffuse freely?| How it usually moves
---|---|---
Small, non‑charged (O₂, CO₂)| Yes| Simple diffusion 35
Large or polar (glucose)| No| Facilitated diffusion 35
Ions (Na⁺, K⁺)| No| Facilitated diffusion or active transport 57

So in the context of a basic biology question, the expected missing word is small.