The correct answer is: A. Ambala
Ambala is a city in the Indian state of Haryana. It is located in the Ambala district, which is in the north-western part of the state. Ambala is the oldest railway station in Haryana, and it was established in 1873. It is also the biggest railway station in the state.
Rewari is a city in the Indian state of Haryana. It is located in the Rewari district, which is in the south-western part of the state. Rewari is not the oldest railway station in Haryana, and it was not established in 1873.
Sirsa is a city in the Indian state of Haryana. It is located in the Sirsa district, which is in the south-western part of the state. Sirsa is not the oldest railway station in Haryana, and it was not established in 1873.
Hansi is a city in the Indian state of Haryana. It is located in the Hisar district, which is in the north-western part of the state. Hansi is not the oldest railway station in Haryana, and it was not established in 1873.