Answer: The line that most creates a playful mood is: “But stand where rocks and pebbles sing”
Step-by-step explanation: The fact that the line starts with the word “but” implies that even though you don’t find what you want or that which you are looking for, there is an option for happines. Also the fact that the writer personifies the “rocks and pebbles” by making them “sing” gives a playful touch to the poem and leads a perfect way to connect with the “seeking of joy” he talks about in the next and final line.