5. I think _______ is one of the most serious problems nowadays. I'm

The correct answer depends on the available options, but considering the given words, the most fitting answer is 'homelessness'. The complete sentence could then be: 'I think homelessness is one of the most serious problems nowadays. I'm...' (the sentence is incomplete). Other options like 'poverty' or 'pollution' could also work based on personal perspective and the full context of the task.