Final answer:
The implied idea is that keeping the Arkenstone would be dangerous for Bilbo, supported by the sentence where Bilbo grows afraid upon hearing Thorin's words about the stone's importance.
Step-by-step explanation:
The implied idea in the passage from The Hobbit is that keeping the Arkenstone would be dangerous for Bilbo. This is inferred from the text where Bilbo hears Thorin's decree about the jewel and grows afraid, pondering the consequences should it be discovered that he possesses the stone. The suspense lies in Thorin's intent to avenge any withholding of the Arkenstone, suggesting peril for Bilbo.
The sentence that best supports this answer in Part A is: "Bilbo heard these words and he grew afraid, wondering what would happen, if the stone was found—wrapped in an old bundle of tattered oddments that he used as a pillow." This clearly shows Bilbo's fear and acknowledgment of the risk associated with the stone.
Both Thorin's obsession with the Arkenstone and the importance he places on it set the tone for the critical value of the stone to the plot and to Bilbo's decision-making process within the narrative.