The correct option is A The duke was a just ruler.
Mostly, adjectives come before the noun that it describes or modifies. However, in very rare cases, they follow a noun, it's confined to archaic and poetic uses and this form isn't used in standard English. In the given statement, the adjective 'just' follows the noun 'ruler', it's incorrect and we need to change it, and hence option D is incorrect. The adjective 'just' should be placed before 'ruler' in order to modify it, like it does in option A, hence option A is correct. The placement of 'just' is incorrect in option B, where it's placed after the noun. Also, 'duke' is a position which should come prior to 'ruler', which is secondary in position, the placement of phrases is not correct, hence option B is incorrect. The adjective 'just' describes the duke as a ruler, it does not modify the duke, hence option C is incorrect.