The effect of the speaker hearing about the statue from the traveller as opposed to seeing it with his own eyes is C. It helps emphasize how the story is a tale that is being passed on to the reader, indicating that there is a message to be heeded.
The message of this story and the poem "Ozymandias" by Percey B. Shelley is the ephemeral character of power and ambition, and the destructive forces of time.
By hearing the story from a traveller instead of seeing it himself, the power of the king is further diminished. The traveller did not see Ozymandias' spledor, only the wasteland where the remains of the statue are. The speaker has not even seen the statue. He has only heard the story, so the impact of the king is further diminished.