In this sentence, the blank requires a preposition that indicates the position of the dog in relation to the table. The word 'underneath' is the most appropriate choice because it specifically means 'directly below' or 'beneath'. The other options, 'down' and 'in', do not accurately describe the dog's position relative to the table. 'Down' suggests a direction rather than a position, and 'in' implies being inside something, which does not fit the context of the dog being under the table.