What causes soils and their horizons to differ?
a.
differences in climates where the soils are formed
b.
differences in plant and animal life in different locations
c.
differences in the layout of the land
d.
all of the above

D. All of the options given can cause soil horizons to differ. Climates can affect the type of soil and the composition (A), the animal and plant life that decomposed to form the soil can alter the composition (B), and the land's layout (C) can have an effect on how the soil forms and its constituent elements.

The answer is joseph is observing the colour of the reaction mixture to see whether proteins are present in the given solution.