Answer:
b) A region of land that is periodically flooded when a river overflows.
Step-by-step explanation:
A foodplain is, by definition, an area of flat land near a river that is often flooded when the river becomes too full.
Now, with this definition in mind let's take a look at the answers:
A. An area that is incidentally flooded when farming or mining operations change the course of rivers.
The definition of foodplain doesn't mention anything about activities caused by humans.
B. A region of land that is periodically flooded when a river overflows
This is correct, since the land is flooded when the river becomes too full (or in other words, overflows)
C. An area that is periodically flooded because humans have altered the landscape
Again, the definition doesn't state anything about human activities.
D. An area where flood irrigation of crops is used, such as a rice paddy
One more time, this is not the right definition, since the definition doesn't state anything about human activities.
Therefore, the right answer is b)