Ice blocks or crushed ice?

“Mrs Heng prepared ice lemonade for her son’s birthday party. She would like the drink to remain chilled with ice for as long as possible. Which type of ice, blocks or crushed, should she use?”

The short answer:
Ice blocks. They have less surface area in contact area with the drink than crushed ice. So they will melt more slowly.

On the other hand, they will also cool the drink more slowly.