A. True
Solid substances can indeed exist in more than one form, a phenomenon known as "allotropy" or "polymorphism." In allotropy, the same element or compound can have different crystal structures or arrangements of atoms in the solid state, resulting in distinct physical properties.
Common examples include carbon, which can exist as both diamond and graphite, and sulfur, which has multiple crystalline forms. These different forms of solids can have different properties, such as hardness, density, and electrical conductivity, even though they are made up of the same chemical elements.