The answer is the third one.
A computer is a common noun.
Common nouns are things that are just as the name suggests, common. An easy way to remember the difference are proper nouns are nouns you capitalize. States and people's names are two examples of proper nouns. Common noun examples include apple, monkey, and cell phone.
The best answer is C because the purpose of a problem-solution essay is to address the issue and talk about how to solve it. This means that the writer will have to talk about what the issue is even if the reader already knows what it is.
