Final answer:
The dominant form of Spanish in Spain is Castilian, which became the standard dialect after the unification of the kingdoms of Aragon and Castille and received cultural influence during the Renaissance.
Step-by-step explanation:
The dominant form of Spanish in Spain is Castilian, which is used as the national language and is the primary language of instruction in schools. This form of Spanish developed after King Ferdinand of Aragon and Queen Isabella of Castille unified their kingdoms, leading to the creation of a standard dialect that was crucial for administration and communication throughout the newly unified Spain. Castilian Spanish also gained prominence through cultural and intellectual exchanges during the Renaissance and has been influenced over time by various historical events such as the Reconquista and the Counter-Reformation.