The ancient city of Petra, located in modern-day Jordan is famous for its rock-cut architecture.
Correct: B
The phrase 'located in modern-day Jordan' is a non-essential descriptive phrase (a participial phrase acting as an appositive). It provides extra information about 'The ancient city of Petra.' As a non-essential element, it must be set off by commas. Since there's already a comma before 'located,' a second comma is needed after 'Jordan' to properly enclose the phrase. Option B correctly adds this comma.