The answer to that question is: D) Invisible
The definition of the antecedent of a sentence is that if a word refers to a previous noun or pronoun in the sentence, that pronoun or noun is its antecedent. Let's eliminate some options. Looked is a verb so it cannot be. All around the room is a general space qualifier and thus this is irrelevant too (it does not refer to Samuel's room for example). Now we find the relevant part, "his shoes". The pronoun is his. Hence, we have that the antecedent is Samuel.
Answer:
the authors attitude toward the subject
