Which of the following models does not require an option key to enable full dual-display functionality?

A.
C20

B.
C40

C.
C60

D.
EX90

Explanation:
The Cisco Codec C60 does not require an option key to enable full dual-display functionality. Dual-display
functionality is native to the C60. The C20, C40, and EX90 all require an option key to enable full dual-display
functionality.
The following option keys can generally be installed on devices that run Cisco TelePresence TC software, such
as the C series and EX series models:
Dual display
MultiSite
Presentation
High definition
Premium resolution
The MultiSite option key enables systems to create ad-hoc conferences without requiring an external multipoint
control unit (MCU). Ad-hoc conferences are on-demand conferences and are limited to four simultaneous
participants.
The Presentation option key, which is sometimes called the Natural Presenter option key, enables systems to
send presentations. This key is supplied by default on new systems. The EX series models include presentation
functionality natively without an option key.The High definition option key allows 720p definition at 30 frames per second. The C40 and C60 support this
feature natively; the C20 requires the High definition option key to support high-definition functionality.
The Premium resolution option key allows higher resolutions up to 1920 x 1080 at 60 frames per second. All C
series models require an option key to support these higher resolutions.

Cisco: Troubleshooting Guide TC 6.0: Option keys (PDF)