Answer:
C. The writing is clearly focused on one topic.
Step-by-step explanation:
The paragraph follows the instructions, it explains in a simple but coherent and logical way the causes en effects of El Niño weather pattern. By doing that, if is focused on the topic, the first sentence tells us what El Niño is and why is is generated. The second sentence tells us the consequences of this weather pattern. As you can see, the cause-effect format is used.
The only thing that can be improved is the lenght of the paragraph, maybe a few mor characteristics of El Niño would improve the essay, like the time of the year it is presented.