The correct chronological order of the conquest of states/empires by the Britishers in the 18th-19th century is as follows:

1. Mysore
2. Bengal
3. Punjab
4. Maratha

Explanation:

1. Mysore:
The British conquest of Mysore took place in the late 18th century. Mysore was ruled by Tipu Sultan, who was a formidable opponent for the British. The first Anglo-Mysore War (1767-1769) ended inconclusively, but the second Anglo-Mysore War (1780-1784) resulted in the defeat of Tipu Sultan. The British, along with their allies, captured the capital city of Srirangapatna, marking the end of Tipu Sultan's rule in 1799.

2. Bengal:
The conquest of Bengal by the British began in the mid-18th century. The Battle of Plassey in 1757 was a significant turning point in British expansion in India. The East India Company, under the leadership of Robert Clive, defeated the Nawab of Bengal, Siraj-ud-Daulah, and established control over the Bengal region. This victory laid the foundation for British control over large parts of India.

3. Punjab:
The conquest of Punjab by the British took place in the mid-19th century. Punjab was ruled by the Sikh Empire, and the British sought to expand their territory further. The First Anglo-Sikh War (1845-1846) ended with the Treaty of Lahore, which allowed the British to annex some Sikh territories. However, the Sikh Empire revolted again in the Second Anglo-Sikh War (1848-1849), resulting in their defeat and the complete annexation of Punjab by the British.

4. Maratha:
The conquest of the Maratha Empire by the British took place in the late 18th and early 19th centuries. The British faced stiff resistance from the Marathas, who were a powerful force in central India. The three Anglo-Maratha Wars (1775-1782, 1803-1805, and 1817-1818) resulted in the gradual decline of the Marathas and the establishment of British control over their territories.

In conclusion, the correct chronological order of the conquest of states/empires by the Britishers in the 18th-19th century is Mysore, Bengal, Punjab, and Maratha.