Option A is correct because the textbook states that spring tides occur 'When the sun, the moon and the earth are in a straight line.' During this alignment, the gravitational forces of both the sun and the moon combine and pull the earth in the same direction, resulting in higher tides. This happens twice a month - during the full moon and new moon periods. Option B describes neap tides where the forces counteract each other, and Option C about coastline indentions is unrelated to spring tides.