What causes the demand curve to shift?

The increase/decrease in price may not necessarily be resulting to the shift of the demand curve. The best answer is the increase/decrease in need. Because, as the need of the product increases/decreases people would not care on the price. Say, there is a recent study that an apple can make people look 5 years younger if you eat 10 pieces of apples a day. Who would not want to look younger? Hence, people will buy more. Hence, the demand curve will shift to the right. Actually 3 answers are possible here, the number 1,2 and 4. 3 is not an accurate answer.