Which of the following is a serial system bus?

A.
PCI 2.1

B.
PCI-X

C.
PCIe

D.
AGP

Explanation:
Unlike PCI and AGP, which are 32-bit and 64-bit parallel buses, PCI Express (PCIe) is a point-to-point serial bus.
Incorrect
Answers:
A, B, D: PCI, PCI-X and AGP are all parallel buses.
References:
Gary Govanus, William Heldman and Jarret Buse, Server + Study Guide, Sybex, Alameda, CA, 2001, pp. 180, 192-193.
Charles J. Brooks, Server+ Certification Exam Cram 2 (Exam SK0-002), QUE Publishing, Indianapolis, 2006, pp. 107-108, 111-112, 114-117.