Final answer:
In the citation Rev Rul. 87-90, 1987-1 CB 198, the number '90' represents the ruling number for the year.
Step-by-step explanation:
In the citation Rev Rul. 87-90, 1987-1 CB 198, the number '90' represents the Ruling number for the year. In this citation, 'Rev Rul' stands for revenue ruling, '87' represents the year, and '90' denotes the ruling number for that particular year. The citation also includes the volume number '198' in the '1987-1 CB 198', which is the volume number in the 'CB' or Cumulative Bulletin. Therefore, the correct answer is option a. Ruling number for the year.