The correct option is (a) Will — "Will you help me cross the road, young man?" ('Will' is used to make a polite request.)
'Will' is used for polite requests in everyday conversation. 'May' and 'Might' express possibility/permission, and 'Shall' is used for offers or suggestions (typically with I/we), not requests directed at others. Examiners expect students to know the functional use of modals.