Question

Fill in the blank with a suitable modal:

How _________ you abuse your elder brother? (could, should)

Answer 1

The correct answer is:

How could you abuse your elder brother?

The phrase 'how can/could you' is a fixed expression that expresses shock or dismay in somebody's actions. Hence this is the correct answer.

'Should' expresses an action that must be done in a particular way. It does not apply to the already fixed expression here.