The best answer would be Letter B - to entertain.
The author merely would like to provide amusement to the readers at his expense. It certainly does not persuade, explain, nor inform anything since it states - <span>But why bother with the descriptions; you’ve probably already grasped the idea!</span> - which could be taken as a witticism.
The right answer for the question that is being asked and shown above is that: "The main idea of the paragraph is this - <span>Volunteering is a good way to gain skills and work experience." It serves as the summary of the whole paragraph and catches the whole thought.</span>
