Which of the following is an example of a human-caused disturbance to an ecosystem?

A. A tsunami destroys a coastal forest and causes flooding.

B. A river is diverted to irrigate farmlands during a drought.

C. After a volcanic eruption, global average temperature lower.

D. The snow cover of a region grows and shrinks throughout the year.

The correct option is B.

Out of all the options given above, it is only option B that include the input of a human activity. Human caused disturbance to an ecosystem refers to those alterations in the ecosystem, which are cause by human activities. For instance, diversion of a river for farming activities will change the natural direction of the river and result in the alterations of the ecosystem.

The other options are incorrect because, tsunami, volcanic eruption and snow are not caused by human's efforts.

It is B. A river is diverted to irrigate farmlands during a drought.