An attribute found in plants but not animals is

Define the following terms:
$(i)$ Unicellular organism
$(ii)$ Multicellular organism

Two plants can be definitively said to belong to the same species if they

Living organisms show responses to external factors like:

Assertion: The number of cells in a multicellular organism is inversely proportional to the size of the body.
Reason: All cells of the biological world are alive.

What is required for a living organism to perform various biological functions?