Concept:The phrase "about to" is used with the base form of a verb to show an immediate future action in the past.Explanation:After "was about", the correct pattern is "to + base verb"."To leave" is the base verb form. It correctly completes the sentence: "My brother was about to leave his house when I arrived."The other options use incorrect verb forms like "leaving" or "be leaving", which do not fit this structure.Answer:C. to leave