Final answer:
Coloring with crayons is the safest and most developmentally appropriate activity for a 3-year-old in a spica cast, supporting hand-eye coordination and creativity without risking the integrity of the cast.
Step-by-step explanation:
The most appropriate play activity for a 3-year-old child in a spica cast would be coloring with crayons. This activity is safe for a child with limited mobility due to the cast and also supports developmental skills such as hand-eye coordination and creativity. Activities like marbles and checkers are not suitable due to the small pieces which can be a choking hazard and the difficulty of manipulating the pieces with limited mobility. Playing with a toy telephone might be safe, but coloring with crayons offers better opportunities for artistic expression and fine motor skill development.
Moreover, coloring is a quiet activity that can keep the child engaged without requiring movement that could compromise the spica cast. It also offers a therapeutic and calming effect which can be beneficial for a child dealing with the discomfort and restrictions of a long-term cast.