The founder of the Avadh Kingdom in the 18th century was Saadat Khan.

Option 1: Murshid Quli Khan was the founder of the Bengal Subah, which later became known as the Bengal Presidency. He was not the founder of the Avadh Kingdom.

Option 2: Saadat Khan is the correct answer. He was a Mughal noble who established the Avadh Kingdom in the 18th century. He was appointed as the Nawab of Avadh by the Mughal Emperor Aurangzeb, and he laid the foundation of the Avadh state and its capital, Lucknow.

Option 3: Alivardi Khan was the Nawab of Bengal, not Avadh. He played a significant role in the history of Bengal during the 18th century, but he was not the founder of the Avadh Kingdom.

Option 4: Sarfaraz Khan was also a Nawab of Bengal, but he was not associated with the establishment of the Avadh Kingdom.

In conclusion, Saadat Khan is the correct answer as he was the founder of the Avadh Kingdom in the 18th century.