Explanation:

- Option 1: Delhi

- B could sue A in Delhi because the impact of the injury may occur where B resides, which is in Delhi.

- Option 2: Dehradun

- B could also sue A in Dehradun, where A, the defendant, resides. This is a typical legal option to file a suit at the defendant's place of residence.

- Option 3: Either (a) or (b)

- ?? Correct Answer

- B has the choice to file the lawsuit in either Delhi or Dehradun, depending on his preference and convenience.

- Option 4: Only (a) not (b)

- This is incorrect because B isn't limited to just suing in Delhi. He has the choice of filing in Dehradun as well.