According to NFPA 101®, Life Safety Code®, a means of egress is a continuous and unobstructed path of travel from any point in a building to a public way. This path is comprised of three separate and distinct parts: the exit access, the exit, and the exit discharge. The exit access leads to an exit, the exit provides a protected path to the outside, and the exit discharge connects the termination of the exit to the public way. Therefore, "exit discharge" is the third component.

A. Exit corridor: An exit corridor is typically a component of the exit access, which is the portion of the means of egress that leads to an exit.

C. Horizontal exit: A horizontal exit is a specific type of exit, not one of the three fundamental components that constitute the entire means of egress system.

D. Vertical exit: This is not a formally defined term in NFPA 101. It likely refers to an exit stairway, which is a type of exit component.

1. NFPA 101®, Life Safety Code®, 2021 Edition. Chapter 3, Section 3.3.178.3, "Means of Egress." This section explicitly states, "A means of egress consists of three separate and distinct parts: (1) the exit access, (2) the exit, and (3) the exit discharge."

2. NFPA, Fire Protection Handbook, 21st Edition. Section 14, Chapter 2, "Means of Egress," page 14-18. The text clarifies, "The means of egress concept is based on the availability of three separate and distinct components: the exit access, the exit, and the exit discharge."

3. Cote, A. E., & Harrington, G. P. (Eds.). (2008). Fire Protection Handbook (20th ed., Vol. 2). National Fire Protection Association. Section 13, Chapter 1, "Means of Egress," page 13-17. This edition also confirms the three-part structure of the means of egress.