Irwin is involved in both passive and nonpassive activities. Which of Irwin's following involvements is a passive activity?

a) Involvement in a trade or business activity in which he does not materially participate during the tax year

b) His working interest in oil or gas wells in which he holds interests directly or through an entity that does not limit his liability

c) The rental of a residential unit that Irwin also used for personal purposes during the year for more than the greater of 14 days or 10% of the number of days during the year that the home was rented at fair rental value

d) His rental real estate activity that Irwin materially participated in as a real estate professional