Almost all animals share the following characteristics except
a. cell walls.
b. diploidy.
c. heterotrophy.
d. mobility.

The correct answer is A.

Animal cells do not have a  cell wall.

A cell wall is the rigid, outermost covering of plant cells and is made up of cellulose.  It is absent in animal cells. The cell wall is visible under a light microscope.

Animal cells are instead covered by a cell membrane. It is made up of lipids, proteins, and small amounts carbohydrates. It is a thin and delicate structure  that can only be seen using an electron microscope.