Final answer:
The geometric shape most suitable to represent 'form' is a Solid, as it is the only option that has three dimensions necessary to depict the concept of form.
Step-by-step explanation:
The geometric shape that is best suited to represent the concept of "form" is (d) Solid. The term 'form' refers to an actual, three-dimensional shape which can be both geometric and organic. In contrast, a point has no dimension, a line has one dimension, and a plane has two dimensions. To visualize three-dimensionality, it is essential to have a solid as it extends in three dimensions (length, width, and height), which fulfils the requirement of representing form.