After lunch, the fattened big fish has a change in both speed and momentum.
Work Step by Step
Note: the book answer to this multiple choice question is "A", indicating that only the speed changes. However, an object's speed cannot change without an accompanying change in momentum. We disagree with the book's answer. This example is done on page 101. The momentum of the system (big fish + little fish) remains constant, but the momentum of the big fish alone changes. The book assumes that after the big fish eats the little fish, it becomes the entire system. However if one thinks of it as "big red clay blob crashes into small stationary blue blob", we see that the speed of the red clay changes, and the momentum of the red clay changes too.