What do tissues combine to form? A. organisms

B. cells

C. organ systems

D. organs

Answer is D organs because cell forms tissues, tissues form organs and organs forms the organ system

D. Organs is correct
Biological hierarchy goes in this order:
Atoms form molecules, molecules form cells, cells form tissues, tissues form organs, organs form organ systems, organ systems form organisms, groups of the same organism is a population, different populations living together is a community, a physical environment of different communities is an ecosystem, a region of a specific ecosystem is a biome, and all the biomes together create the biosphere. :)