The detail from "Translating Grandfather's House" that most clearly shows that the speaker's memories are real is:
B. The roof Oscar Jartín / Tumbled from one hot Tuesday…. (lines 22–23)
This line suggests a specific event that happened on a particular day, indicating that the memory is grounded in a concrete reality rather than being vague or abstract, which helps to convey the authenticity of the speaker's memories.