Final answer:
Rafi is most excited about going on a whale watch, which is indicated by his enthusiastic response to the announcement and expression of his long-standing wish to experience a whale watch. Option b.
Step-by-step explanation:
Rafi's excitement is most specifically directed towards the prospect of going on a whale watch.
His enthusiastic exclamation about the whale watch and stating his long-standing desire to participate in one are clear indicators of his anticipation.
Although the field trip includes spending time with friends and possibly enjoying the subject of biology, the text provided puts focus on the whale watching experience, making choice B (Going on a whale watch) the correct answer to what Rafi is most excited about.