Final answer:
Robbie could have written a song for Julia for various reasons, such as love, to impress, to make her laugh, or to thank her. The most likely reason based on common perceptions might be love, but without further context, this is speculative.
Step-by-step explanation:
If Robbie wrote a song for Julia, without more context from the situation, it is difficult to determine the most likely reason with certainty. However, the common reasons someone might write a song for another person include expressing love, trying to impress them, wanting to make them laugh, or as a way of saying thank you for something. Considering that songs are often seen as a romantic gesture, one might be inclined to choose (A) He is in love with her as the most likely reason. Nonetheless, without additional information, this remains speculation. Each option reflects a possible motivation, but the true reason depends on the nature of Robbie and Julia's relationship and the content of the song.