Answer:
Personification.
Explanation:
Personification is a figurative language where a writer attributes human qualities to a non-human entity, an abstract idea or element. It allows the abstract idea or theme to have human characteristics, and make it seem like it is like a human.
So, in the given sentence, the sun is personified as a living human being that "peeked its head over the horizon". This act of "peeking" is a human quality where the sun is like a human being capable of human actions. Thus, the figurative language used in the given line is personification.
