Identify the complex sentence from the options below:

Correct option is B

He went to his father's native place. = It is a simple sentence.
He succeeded although his success was not expected. = It is a complex sentence.
He was a mere boy but he offered to fight the giant. =It is a compound sentence.
He must have done his duty for he is a conscientious man. = It is a compound sentence.
So correct answer is option (b) He succeeded although his success was not expected.