Final answer:
Deep structures in sentences are composed of propositions, which are the underlying meanings separate from the sequential arrangement of words or visual representation by images.
Step-by-step explanation:
Deep structures are assumed to be composed of propositions. Unlike words and images that convey information through a physical representation or sensory perception, propositions represent the underlying, abstract meaning that composes the deep structure of sentences. While words are placed sequentially and can include stylistic variations to convey different meanings or emphasis, and images augment the information visually, propositions themselves are the semantic building blocks that constitute the meaning behind the sentences and phrases, independent of the language's superficial grammatical structure.