Final answer:
The line indicating the speaker's awareness of the temporary nature of joy is C. "I knew this moment wouldn't last forever." This line explicitly recognizes the transient aspect of the joyful experience.
Step-by-step explanation:
The line that tells the reader that the speaker of the poem knows his joyful experience is temporary is C. "I knew this moment wouldn't last forever." This line directly communicates the transient nature of the speaker's joy, contrasting with the other options that either describe an ongoing state of joy or a particular joyful scene without mention of its duration. While line A, "I danced in the rain with a carefree heart," and line B, "The sun was setting as I laughed," suggest moments of joy, they do not explicitly indicate that they are temporary. Line D, "Joy was my companion all day long," implies a sustained experience rather than a fleeting one.