A yellow pennant-shaped sign placed on the left side of the road marks:

Correct answer: A – The beginning of a no-passing zone
The NO PASSING ZONE sign is a pennant-shaped warning sign with its point directed to the right. When used, it is installed on the left side of the roadway at the beginning of a no-passing zone identified by pavement markings, DO NOT PASS signs, or both. Other sign shapes are reserved for stop, railroad, and wrong-way messages.
Source: 2009 MUTCD — § 2C.45, “NO PASSING ZONE Sign (W14-3)”, Wyoming Rules of the Road Driver License Manual — “Traffic signs”


