According to NFPA 101, Life Safety Code, the fire resistance rating for the enclosure of a vertical opening is determined by the number of stories it connects. For new construction, if a vertical opening, such as a stairway or shaft, connects four or more stories, the fire barrier enclosing it must have a minimum fire resistance rating of 2 hours. Since the building in the question is six stories high, any floor opening connecting these levels falls under this requirement.

A. 1-hour: This rating is the minimum requirement for enclosures of vertical openings that connect three or fewer stories, which is not applicable to a six-story building.

C. 3-hours: This rating is typically required for fire walls separating different buildings or dividing a single building into separate fire areas, not for standard vertical opening enclosures.

D. 4-hours: This is the highest rating generally specified in codes and is reserved for fire walls under conditions of severe fire exposure, far exceeding the requirement for a typical shaft enclosure.

1. NFPA 101, Life Safety Code, 2021 Edition. Chapter 8, "Features of Fire Protection," Section 8.6.5.2.1, states: "Where the vertical opening connects four or more stories, the fire barrier shall have a fire resistance rating of not less than 2 hours."

2. Cote, A. E., & Harrington, G. P. (Eds.). (2021). Fire Protection Handbook (21st ed.). National Fire Protection Association. Volume 2, Section 16, Chapter 2, "Vertical Openings and Communicating Spaces," page 16-26.

3. DiNenno, P. J. (Ed.). (2016). SFPE Handbook of Fire Protection Engineering (5th ed.). Springer. Chapter 61, "Fire Safety in Buildings," Section 61.4.2, "Vertical Openings," page 2698. (This section discusses the principles of protecting vertical openings, aligning with the 2-hour requirement for buildings of this height as specified in model codes).