The correct answer is 4.
Problem and solution is a pattern of organization in a text. Here, an information is expressed as a problem or a dilemma and a solution or attempted solution is looked for and presented. This kind of text is frequently confused with a cause and effect type of text. However, the cause and effect text does not provide the solution, that's the difference between these two types of texts.
This kind of text often uses signal words to indicate that information in a passage is ordered in the problem and solution pattern. Such words are: solution, propose, answer, issue, problem, remedy, fix, prevention etc.