For language is arbitrarily produced by the imagination and has relation to thoughts alone; but all other materials, instruments and conditions of art, have relations among each other, which limit and interpose between conception and expression. The former is as a mirror which reflects, the latter as a cloud which enfeebles, the light of which both are mediums of communication. Which detail provides more information to support Shelley's idea that language, unlike other forms of expression, comes from the imagination?

A: "For language is arbitrarily produced by the imagination"

B: "and has relation to thoughts alone"

C: "The former is as a mirror which reflects, the latter as a cloud which enfeebles"

D: "both are mediums of communication"

The quote is incomplete. It should read: "For language is arbitrarily produced by the imagination, and has relation to thoughts alone; but all other materials, instruments and conditions of art have relations among each other which limit and interpose between conception and expression. The former is as a mirror which reflects, the later as a cloud which enfeebles the light of which both are mediums of communication. Hence the fame of sculptors, painters and musicians ... has never equaled that of poets ... "

A Defence of Poetry - Percy Shelley

In my view the correct answer should be B: "and has relation to thoughts alone".

The reason being that as Shelley explains later, he considers that language has a direct, unique and exclusive relation with thoughts. Logically, thoughts are a direct product of the imagination, whether they are spontaneous or a result of external stimuli, and if according to his logic language is intimately and exclusively related to them it follows that language comes from the imagination. For Shelley as a poet, because language comes directly from imagination without the mediation or interference of any kind, poetry is the purest form of art.