Five animals are moving toward the river.
Here's a detailed answer:
1 duck noticed 4 elephants as it was going toward the river. Hence, 1 animal (the duck) is moving to the river.
The 4 elephants saw 2 monkeys going walking toward the river. Now comes the tricky part. You may think that each elephant saw 2 different monkeys walking toward the river. So you would conclude that 4 x 2 = 8 monkeys were going to the river.
However, the task does not explicitly say that each elephant saw DIFFERENT monkeys. In fact, you can logically infer that the elephants saw the SAME 2 monkeys. Hence, we only have 2 additional animals (monkeys) moving toward the river.
Since every monkey was carrying 1 dove, the 2 monkeys were carrying 2 doves in total.
Summing up, we have 1 duck, 2 monkeys, and 2 doves moving to the river. 1 + 2 + 2 = 5 animals.