Why does a star of
average mass expand
into a red giant?

When a star expands, the surface of the star becomes cooler as well as redder. Hence, forming the red giant.

Explanation:

If all the hydrogen which is present in a star gets fused into helium then an increase in the core temperature can be seen along with the core contractions. As a result of these core pressures and the core temperatures, the helium gets fused into carbon through a process known as the triple alpha process. As a result, the star begins to expand whereas its surface becomes cooler and redder which leads to the formation of red giant.

oxygen +1 most likely

only about ten percent of the energy at one trophic level is passed on to organisms at the next trophic level. the remainder of the energy is used in life processes or lost as heat.