Fill in the blank with a suitable Modal from the options given below:

How ________ you shout at your elder sister ?

could

In the given sentence, the modal "could" is the most suitable choice. It expresses a possibility or ability in the past. The sentence is questioning the appropriateness of shouting at the elder sister, implying that it was possible or allowed.

 "Could" implies the ability to do something in this context and is more appropriate than the other modals like "shall," "would," or "must" for this specific question.