Final answer:
The level of abstraction in Constantin Brancusi's Bird in Space is most similar to that of his earlier sculpture Sleeping Muse I, with both emphasizing a move towards simplification and essence over realistic representation.
Step-by-step explanation:
The level of abstraction in Constantin Brancusi's Bird in Space is most similar to the level of abstraction in his earlier sculpture, Sleeping Muse I. Both works represent a significant departure from realistic representation, focusing on simplified forms and clean lines.
While Sleeping Muse I abstracts the human form to a streamlined depiction of a head, reducing its features to the most elemental shapes, Bird in Space takes abstraction further by stripping away all bird-like characteristics apart from an elongated form suggesting flight and ascension.
This comparison highlights a progression in Brancusi's work towards purity of form and an exploration of essence rather than a literal representation.
By contrast, The Kiss and The Thinker are more representational and less abstract in their depiction of the human figure and emotion. The Gates of Hell is a complex, narrative-driven piece with many figures and a high level of detail that contrasts starkly with the simplicity of Bird in Space.