$\text{Option A is wrong}$ beacuse set young people contain yes island people but it did not conatin only yes island ; it may have more kind of people which can speak false also.
$\text{Option B}$ is also wrong bcoz third statement cannot determind by first $2$ statements .
$\text{Option C is correct}$ because all yes island people speak truth and some young people are the citizen of yes island so it is straightforward that some young people will definitely speak truth ..