Step 1: Differentiate between raw clay and fired ceramic.
The question asks to define "clay," which refers to the raw material in its natural, plastic state.
Step 2: Analyze the options.
Option (A) accurately describes the physical properties of unfired clay: it is a type of earth that is moist, stiff (can hold a shape), and tenacious (sticky and cohesive). The other options (B, C, D) all describe clay *after* it has been fired in a kiln, at which point it becomes ceramic.