Final answer:
The correct form of the indirect object pronoun is lui.
Step-by-step explanation:
The correct form of the indirect object pronoun to complete the sentence is lui.
Here's the corrected sentence: "Oui ! Je lui ai peint la plage et le soleil couchant."
In French, the indirect object pronoun "lui" is used to refer to "ton père" (your father) as the recipient of the painting. Other examples of indirect object pronouns include "me" (to me), "te" (to you), and "nous" (to us).
Learn more about Indirect object pronoun in French