Final answer:
All provided options, a. Revenue Ruling 2004-33, b. RR 2004-33, and c. Rev. Rul. 2006-33, 2004-1 C.B.628, are proper forms for citing a Revenue Ruling, varying by context and citation style.
Step-by-step explanation:
The proper citation to a Revenue Ruling depends on the specific style guide being followed. However, the options provided can all be considered correct under different circumstances or formats that are commonly used. For instance, a. Revenue Ruling 2004-33 is a straightforward way of referencing a ruling, mentioning the year followed by the specific ruling number. Option b. RR 2004-33 uses an abbreviation for Revenue Ruling, which can be acceptable shorthand in certain contexts. Finally, c. Rev. Rul. 2006-33, 2004-1 C.B.628 is an example of a full citation that includes the Revenue Ruling, the year, the ruling number, and the volume and page number of the Cumulative Bulletin where it can be found. Therefore, the correct answer is d. All of these choices are proper forms for citing a Revenue Ruling, depending on the context and citation style being used.