The correct answer is option 3, Ajanta Caves. The Ajanta Caves are a group of rock-cut Buddhist temples located in Maharashtra, India. They were built over a period of centuries, with the earliest caves dating back to the 2nd century BCE. These caves were used by monks as meditation halls and monastic complexes. The Ajanta Caves are known for their exquisite rock-cut architecture and intricate paintings, which depict Buddhist stories and teachings. The caves were rediscovered in the early 19th century and have since been recognized as a UNESCO World Heritage Site. Thus, out of the given options, Ajanta Caves are the oldest monument.